Difficult to say for sure, bet I bet someone will definitely try. G1's hardware might not be up to par for running X10's UI, so those ROMs might be very laggy.
Unfortunately I think this will be the trend for all future phones. The fact that those flashy new UIs are usually proprietary (read: closed source) means that the community devs will not be able to optimize them to run on older hardware. At best they will port them to run on G1s and MT3Gs, but those ROMs will not run well.
Personally, I'm more than happy to continue using my G1 with stock OS (properly cyanogenized of course.) I just wish HTC/T-Mobile/Google/whoever hurry up and upgrade them to 2.x.