Many of the older device owners and early adopters will be glad to hear this one - I've talked to a bunch of Google folks in charge of developing Android at the Google I/O, and they told me that technically it would be possible to put Froyo on any device on the market now, including the G1.
Since I was trying to specifically concentrate on the G1s due to those devices having the lowest specs among all Android phones, a lot of my questions were asked with this Android dinosaur in mind.
During my discussions, I was told that even though the G1 had smaller internal storage capacity, it would be possible to package up an update without the extra Google apps, and then install them later on a different partition (or so I understood). In fact, Cyanogen already kind of does this whole Google app separation in his CyanogenMods but that is due to a C&D he had received. Either way, technically it's already been shown possible. Also, when I brought up Cyanogen, the Google devs had no doubt he'd be able to pull off a Froyo update for the G1, regardless of whether the carrier would want to bring it to you or not.
Such an update would not be OTA and would wipe the phone, which is not entirely unheard of, considering the last Sprint HTC Hero update does exactly that.
Furthermore, I was told that there are unofficial test builds running Froyo on G1s within Google, and they're speedy as hell compared to the previous versions, making up for the slower CPU and lower specs in general. Could it be that your old G1 will be able to shine brighter than it had ever before?
Of course, as far as official releases go, it will be entirely up to the carriers and the manufacturers to package up and offer the Froyo updates to their customers.
15 minutes until the keynote - we should know more very-very soon!