IT'S HERE! Android 2.1 on the Eris

All credit goes to:

It’s a very simple process and I, as well as a coworker, have successfully done it. Well worth the update.

Step 1: Download the 2.1 package
Step 2: Put the package .zip file on the SD card  of your phone. Leave it in its .zip format.
Step 3: Turn off phone.
Step 4: Turn on phone by holding the power and send buttons at the same time.
Step 5: Press the volume down button, when prompted, to enter hboot
Step 6: When asked to update, press the trackball (this is the Action button)
Step 7: When asked to reboot, press the trackball.

That’s it. Have fun!

Oh yeah, I’m not responsible if you break your phone.