That's exactly what happened to me with my Android phone. I have one of the first ones that T-Mobile offered. The latest OTA update for it was 1.5 and there are apps I need for school and other things that only run on 2.1 and above. They released a 2.1 ROM for the phone a while back, but the performance is so laggy and crappy it isn't worth using it anymore. Ridiculous things, like if I type in someone's name to send a text message to, it will take 30 seconds for it to pull up the contacts list and then another 30 seconds for it to scroll to the person's name. And if you dare touch the screen or any button while it's "thinking" it slaps you right back to the previous screen and you have to start all over.
And it's because they came out with newer phones with faster processors and more RAM. Mine physically can't handle the demands of the 2.1 Android OS.
I really hope Apple doesn't allow that kind of thing with the iPad and future IOS upgrades. That's the biggest reason I won't buy an Android tablet.