If you are even considering rooting the phone, I would hold off on the update until you decide, as I don't think you can root after you OTA to 2.1.
Just to clarify, if you receive 2.1 over the air on a 1.5 Eris, you can still root, using the same exact procedure. This is exactly what I did - I waited patiently for 2.1, found some annoying bugs, and rooted (which ended up fixing them.)
go to setting and then phone, and then click updates i believe and then you can do the 2.1 ota. You can root the 2.1 ota it has a bootloader of 1.47 where the 2.1 RUU or leak version has a BT of 1.49 and is locked
Yes, this is correct.
To be precise, if you start the phone in the bootloader (hold down "volume down" while powering up the phone) there will be the HBOOT version at the top.
If the version is 1.45, 1.46, or 1.47, your Eris is rootable.
If the version is 1.49.2000 and the top line ends with S-OFF, you are already rooted.
If the version is 1.49.0000 and the top line ends with S-ON, you cannot root your phone using the typical procedure.
If you had an Eris running 1.5 and received 2.1 over the air, you have bootloader 1.45, 1.46, or 1.47.
There is some really good information here:
Basic Root Troubleshooting Guide for the HTC Droid Eris - Android Forums