When is 2.1 coming to the G1?
Right now, if you root, which I recommend you do, and it works pefectly, even the live wallpapers work, although not as smoothly as on a Nexus 1/Droid.
Officially.....maybe never? It's unclear and seems unlikely. Here's why.
The G1 has a limited amount of system memory for the OS. Android 2.1 is bigger than can fit on the amount of memory that the G1 has available.
However, there's a partition in the system memory that is used for OTA updates.
If you have root, you can install a new SPL (secondary program loader) called DangerSPL, which resizes those partitions, giving the G1 much more room for the OS. The new 2.1 Cyanogen ROM requires the DangerSPL because of the size of the ROM.
So what you do is, get root, flash a new radio image (the 26I radio), flash the DangerSPL, then flash the new Cyanogen ROM and then you'll have full Android 2.1 on your G1.
It sounds more complicated than it is, it's actually a very simple process.