Here is why I don't think we will ever see an update....
Eris runs 1.5 (Cupcake), ported to support CDMA in lieu of having 1.6 (Donut). Since Exchange support was not built into the Android OS natively in 1.5, HTC implemented their own.
Going to 2.x would be a major change in respect that they would need/want to wipe out the HTC Exchange support to make way for the native Exchange support. Either that, or they would have to remove/hide/block the native exchange support in the 2.x OS to continue to allow the HTC version to run.
Replacing HTC's exchange solution with the native one in 2.x would mean that the user's exchange relationship would be wiped out, and they would have to set it back up the native support. This would be viewed by VZ as a majorly bad customer situation that likely has no clear method of making it a smooth or transparent transition.
For most of us on this board, no big deal, we just set up our exchange credentials in the updated OS. For many though, this would be a difficult task likely resulting in an onslaught of calls/visits to Verizon. I just don't think VZ would allow this.
I think that if this update is made available, it will come in the form of a tethered upgrade, agreeing to the fact that you will have to re-setup your device.
Just my thoughts.