I'm guessing (keyword guessing) that the update for the Eris will be much like the update for the Sprint Hero. A quick patch to fix a few glaring bugs, but not a new code base. This makes the most sense, as whether we like it or not, Vzw/Moto's responsibility is to get the device stable with it's current features.
Let HTC finish their work on 2.x in house and it's a good possibility we all will get a much more stable and working version of Sense 2 in the near future.
The one thing about the whole situation that does bother me -
Google Maps (a highly publicized feature of all 3 CDMA HTC phones) is no longer supported by the developer on a phone that's only a month old. HTC owes us all some patch to make Google Maps updates available on a 1.5 system, or Android needs to revise their position and continue to maintain the 1.5 and lower versions.
But like Dad always said - It's good to want things.