USB 3.0 seems a more natural upgrade path for tethered iOs devices than LightPeak.
Therefore, I expect we'll see Apple introduce USB 3.0 across their ENTIRE product line -- as soon as its introduction won't mean sacrificing other design concerns like battery life, ubiquity of ports or overall simplicity.
Since an add-on card would certainly affect simplicity, the ball's in Intel's court.
I'm not so sure that Apple will go to USB 3, though a lot has to do with the timing of when LP is available, and how quickly USB 3 gets picked up in the mass market.
I believe that USB 3 is going to cause confusion (which Apple avoids like the plague) with no real benefit to the mainstream market. Most external HDs in use today are USB 2. So, for example, if I buy a new Mac with this "Faster" USB 3 port, then I will want my external HD to work faster when plugged into this new-fangled USB. But, it won't - since it's most likely USB 2. So I go back to store, and now I'm told that i have to buy a new HD - a 'special' external HD. Now my new Mac, that I bought to take advantage of the new USB 3 thingy, costs more.
This (hypothetical) scenario won't apply to everyone, just the people who know just enough about computers to be dangerous. They kinda know about transfer speeds, but not enough to understand the difference between USB 2 and USB 3. Those of us who understand a bit more, and I'm not claiming to be an expert - just an advanced amateur, have already moved our external USB HDs to eSATA, or (in my case) FW800 if improved transfer speeds were actually worth spending money on.
One of the things that Apple makes a lot of money from is the perception in the market that a Mac is easy to use. And, Mac users can get pretty upset when simple things don't work out (just check some of the threads here at MR, eh?). Conversely, if you listen to Window's users - you frequently hear them sigh, and mutter about how "it's just one of those computer things". Windows users (and yes I'm stereotyping, big-time) will more likely put up with the USB2/USB3 confusion because they're resigned to things not quite working out. Or, that buying a new computer means an extra trip to the store to buy the other hardware to make the upgrade actually work. But when your computer cost $330 less than a Mac, spending the extra $115 doesn't hurt so much.
Plus, USB 3 isn't sexy.
While Light Peak is also going to cause confusion to Mac users (the mass market ones, which is where the money is) .... Apple can make LP sound
Sexy .... (wish I knew how to make that word blink). One cable to connect every thing. Even the name "Light Peak" sounds
Sexy. Seriously - if you were designing a marketing campaign, would you want to work with USB 3 or Light Peak?
My prediction is that Apple will debut Light Peak on the Mac Mini and Mac Pro first. The Mac Mini will be part of package that could include either an Apple branded monitor, or working with a 3rd party monitor maker. The Mini will have two LP ports, and that's it for ports. The Monitor will connect to the Mini by LP, and the monitor will act as a powered hub for the legacy USB/FW/etc. By debuting it on a Mini, Apple gets all the tech press excited which will then get picked up by the mainstream press. But they get to iron out the bugs on a relatively low volume product. When the portables are introduced with LP shortly after, there will be a pent up demand from buyers who have been whipped into a frenzy by the reviews that they have read. It will also give the peripheral makers time to start ramping up their product lines.
I think the Mac Pro will also see it, perhaps just as an expansion card, though I think it more likely that it will be a complete overhaul of the system. Mac Pro users are the ones who can make the most use out of LP. External storage is getting messier and messier to manage, with hubs and expansion cards, and replacing the 2nd optical drive with one more internal HD. It's messy, and Apple hates messy (like the plague - see above).
The new Mac Mini didn't really need that big of a case makeover, like they one they got last year. Apple could have kept selling the old case with new innards for a lot time to come. Simply popping in a black plastic top would have bumped Mini sales by 30%, I'll bet. I think Apple spent the money on designing the new form factor because they knew they were going to be updating the innards, and that the new innards were close enough to the old layout that the could keep using the new case.
I think the last update to the Mac Pro was just the opposite. That the new Mac Pro with Light Peak is going to change the form factor so much that they will have to completely redesign the case, so there was no point redesigning it for the last update to 12 cores.
But I really don't know. I could just be whistling in the wind here. But I am excited about Apple could do with LP. It's been awhile since Apple had a chance to turn the computer industry on it's ear, and I think LP could be a technology that Apple could have a lot of fun with.