We're sure seeing a lot of that around Android forums, not just the Eris.
Remedies are dependent on various factors, such as what version of 2.1 you are running (the OTA or a leak), whether or not your device is rooted and overclocked, the condition of the battery at the time of the rebooting, whether or not it's on the charger at that time, whether or not the device is loaded up with apps and/or what apps you have in there that could be causing the problem (we've seen task killers as the culprit), etc.
My first reaction to that problem, and many others if I had them, would be to do a factory data reset following a backup regimen via Google contacts and calendar, etc.
I'd go from there.. but if that has already been done, we'd need to know more about the phone and the circumstances; there are many things to try before returning the device to Verizon for a replacement or repair (which is the first thing many think of and have success doing that).