This is a very good question, one that gets raised a lot and one which I feel (no disrespect intended to the previous posters) has not been adequately answered.
It is technically possible for Mac OS X to run on any x86 hardware, but their are a multitude reasons why Apple does not want this to be the case.
I have outlined the reasons in full below, at the end of each paragraph I have written a summary in case you don't have time to read the whole thing (but you are welcome to if you choose).
Apple makes practically all of its money on hardware, however their software adds immense value to their hardware. Because of this Apple views itself as a Software company. Think about this: Apple's hardware (internally) is almost identical to generic PC OEM boxes. What distinguishes them is the Mac software. If that distinguishing feature goes away, Apple will sell less hardware. Bearing in mind my opening statement, it becomes very easy to work out why this poses a problem.
In Summary:
[1] Whilst Apple makes money on hardware, without software, that hardware would not be as enticing proposition.
Microsoft don't sell that many retail copies of Windows (relative to the amount they sell in total). Most are bundled and sold to PC OEMs very cheaply. That market is very hard to crack (hard (Microsoft have established relationships and partners, they can offer a very reasonable price, thanks to their economies of scale) On top of this their aren't huge margins in selling to OEMs (unless you sell a lot of copies).
In Summary:
[1] The Retail market isn't huge, but there are fairly decent margins.
[2] The OEM market is bigger, way bigger, but there are only small pickings unless you say, supply every manufacturer in the world (like Microsoft). Market share is king.
Microsoft would leverage their huge economies of scale to easily undercut Apple's OEM pricing. The PC manufacturers would then either not offer Mac OS X or offer it as a more expensive build-to-order option (passing the cost onto the customer). For many Windows is “good enough” and they wouldn't see any point paying more for Mac OS X (something unfamiliar to learn).
In Summary:
[1] MS wouldn't take kindly to competition (they never have) and would easily be able to undercut any newcomer to the OEM market on price.
[2] If bundled by PC manufactures Mac OS X would look more expensive alongside Windows. This would have a detrimental effect on sales.
Where Microsoft makes a lot of their money on Windows + Office is through their deals with business (though large user licences and support etc.). Another area is the server market. These are ares where Microsoft has existing accounts set up and harvesting money. Microsoft would fight tooth and nail to keep them. Again, another tough nut to crack. It is unlikely Apple would be able to compete on price and the cash outlay required to build up the same sort of relationships Microsoft have built up over the years would be vast. It may also ultimately be fruitless. Businesses are sometimes reticent to change from the familiar (for very good reason). For example, if a company switched from using Office to iWork Pro (or whatever), even if iWork Pro worked better, they would still have to retrain all their staff).
In Summary:
[1] Microsoft have many well established links with large corporations around the globe. To try and build up the same sort of relationships would take a lot of time, resources and money. It may ultimately prove fruitless.
So whilst Apple could go down the licensing route, I hope I have painted a picture a lot more complex than many like to make out make out. Apple would have to charge about $200 for Mac OS X, at least. They simply couldn't just put out a $129 DVD for someone to put on their Dell. $129 represents the value of the upgrade (from the OS you brought when you got your computer), not the value of the full product. In other words the money made from selling hardware subsidise the development of the operating system, which in turn helps to sell more software.
In Summary:
[1] $129 is not the true cost of Mac OS X, this is the cost to upgrade to the next version. The cost of the whole version is included in the cost of your machine.
Then Apple would have to include many of the anti-piracy measures that Microsoft imposes, unless they want to lose money hand over fist. By making everyone buy a Mac to get Mac OS X, Apple at least recoups some of the costs of software development, even if the user chooses to pirate a copy of the upgrade (like Leopard).
If you are making not even a cent on the hardware, then you must ensure you make money on the software. This means doing all you can to prevent piracy and to achieve that you must make customers prove they have actually purchased your software. Rightly or wrongly, most consumers don't see value in software. More people would feel guilty about stealing MacBook Pro than pirating commercial software. Piracy of Windows is rampant in some countries. This hurts Microsoft now, but would hurt a smaller company like Apple even more.
Apple would also have to ramp up AppleCare so the staff could support all sorts of curious hardware combinations and retrain all their Genius Bar and Apple Store support staff. All of this would cost.
In Summary:
[1] Other costs are involved in licensing. Apple would have to ensure customers weren't pirating (as Apple now has to make money from the software, not the hardware).
[2] Apple would also have to retrain all staff in AppleCare and Retail to deal with non Mac hardware.
Maybe one day, but at present I think would be a total disaster for the company from a financial perspective. Maybe Apple could make it work, but they would need Microsoft's position to weaken first. At the moment Microsoft have so much of a stranglehold on the market it is foolish trying. Whilst you can argue over the merits of Mac OS X vs Linux all you like (personally I think Mac OS X in more consumer friendly) the various Linux distros still haven't really penetrated the desktop space, even though they are given away for free.
In Summary:
[1] Giving away an operating system has not weakened Microsoft's stranglehold on the desktop market, so it is unlikely charging more for one will.
[2] Never say never, Apple may try again one day, but not yet whilst Microsoft has such a dominant position in the market.