Micro transactions, to the best of my knowledge, for the most part have only been seen in MMO games (most of which have failed). The Sims 3 and now NCAA 10 (both EA) are the only non online games that I am aware of where micro transactions are a part of the game. I suppose you could also count some map packs from certain FPS games. I feel that these are also a ripoff, but more acceptable considering they are post release, not to mention online.
The reason I believe we could be headed down that path is this. Who would have ever thought we would be given an option to buy extra pipeline states, extra stars, or a recruiting advisor. EA already makes you pay for extra items in the Sims 3. Simple fact is if you give a little then the other will probably try and take a little more. Who knows how many reitterations of NCAA and Madden it could be down the road before we start paying for rosters but I still think it's a real possibility.