I install (and uninstall) games pretty regularly. To me these have the potential to be the big differentiator. It will be interesting to see if we end up with fragmentation (think DS vs. PSP vs. iOS) or ports of most major releases as long as each platform has decent market share (Xbox 360 vs. PS3).
It seems that hardware-wise and market share-wise, we're trending towards seeing major releases on at least iOS and Android. What's interesting to me here is that (1) so far it seems like indie developers are playing a larger role on the mobile gaming scene, and they won't necessarily have the resources to do a launch on both platforms until their games become hits, and (2) for whatever reason it seems like the Android gaming market (and app market in general) is very anemic compared to iOS, despite having a large install base. (This is just my subjective opinion based on using an Android phone for the last few months.)
As a side note, I do agree that in general desktop OS choice is less important than it used to be, but I think that it's due more to the democratization of data than it is to the rise of web apps. Getting a Mac file to open on a friend's PC used to be pretty difficult for the average user.