Looking at this from HTC's and Verizon's point of view, I see no motivation for them to spend months developing a 2.2 update for a phone that uses older hardware and which is not one of their top-tier Android handsets. I would think that developers are working on FroYo updates for people who have root, but I seriously doubt that there will be official updates released.
I think that Verizon and HTC really wanted to make sure that Eris users were not stuck at 1.5 but received an Eclair update. They have done that. I can see them coming out with bug fixes for 2.1, but not 2.2.
I think Verizon usually gives one update prior to end of life, which for the Eris, I think is June. If they EOL the phone, there won't be any further official updates, though we will probably get fixes for a few months. I hope so anyway!
VZW will prolly get a 2.2 update out for the Eris. Remember, the standard upgrade is 20 months so expect support for that long at least. Android is easier to test than bb, better internal documentation and change logs from Google, so that makes VZW's job easier. I say yes, it'll get upgraded. But ill be onto a different phone by then.
I thought they said at the 2.2 announcement that the Eris is capable of it, but I didn't see anything first hand. I guess you should go look up the transcript of the announcement and see what they said, if anything.