I think I can safely give you an answer.
You probably would be able to install Lion on your old system.
But you would be breaking the license agreement to do so. The only legal thing to do is buy a copy for the older machine.
I would second guess that. Unless you can find fine print stating that this is against their policy, I would say that it's a fair bet you'll be able to install on all your Macs without fear of violating any terms of use.
Apple has been very adamant that an App legally downloaded through their App store can be installed on all your Macs. So whether you get Lion by ponying up the $30, or through the up-to-date program for getting a new Mac, i don't think it will make a difference. Since Lion will be tied to your iTunes account, most likely as a "purchase"', I think that would give you the right to install multiple times on multiple machines. Of course I'm guessing here. But nothing I've read contradicts this so far.
Perhaps a better way to look at it is this.....After Lion is released and all Macs ship with Lion built in, if a user purchased a new machine, would that user be able to install that included copy on all of his/her Macs? I know that that was not the case with every Mac OS before Lion as one license = one machine, but things have changed now and the policy is a bit fuzzy and not as clear cut. I'm curious to know more about this particular scenario.
Any factual clarification on the subject would be helpful though. I could be wrong about all of this, but if it were me, I would just install it on all my Macs, since I legally own a copy, and Apple now seem to just care if you do own one copy per household for any of it's software distributed digitally through the their App stores.
