Interestingly, Verizon is rolling out a new update right now, and Exchange 2010 support is one of the things that the update is meant to add/fix.
See this page:
Software Update: DROID Eris by HTC
The first PDF on that page lists Exchange 2010 support as a feature.
People on the forum here are reporting that they are getting this update. Until somebody reports the location of the update, there is no way to manually add it until your phone is randomly chosen by the servers to be updated. It usually takes about 10 days to 2 weeks to update everyone, if past history is followed.
If you cannot wait, you can try the application Touchdown. I've not used it myself, but they say that they support Exchange 2010. That app costs a lot (the key app is $20) but there is a 30 day trial. That may get you through until you get the OTA update.
See
https://market.android.com/details?id=com.nitrodesk.droid20.nitroid and
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