XDA Developers have managed to get their own Android 2.1 built on to the Droid Eris. There's a few things to mention about the update though in that with this particular build you are only able to go from Android 1.5 to 2.1 by putting a zip file on your microSD card. If you have previously rooted your phone and installed a 2.1 update then currently there is no way of jumping back to the standard Android 1.5 to perform this... so don't try.
This particular version has also fixed a bug that required you log in each time you rebooted the phone.
For more details and the exact steps to take when updating your phone follow along at this thread on XDA Developers as well as checking up on the finer details over at AndroidMobileOS.
As always... if you run these sorts of updates then do it at your own risk as potentially it can kill your phone.