IIRC, Google has released the Android 2.2 kernels to their open source project, but it will take time for homebrewers to polish them up for tomorrow's unsupported handsets.
Every major Android phone is likely to get the update, but it'll take time and betas.
//EDIT: In many cases, the homebrew community DOES provide ROMs before the carrier. The Samsung Moment, HTC Hero and HTC Droid Eris were all given Android 2.1 by the community before their respective carriers pulled the trigger.