If you have Rom Manager there is an option in it to flash back to stock frf91. I t says rooted and nonrooted options.

Another link:

"Going back to 2.2 Stock ROM (for anyone who want to revert back and forth b/w nightlies and stock):
It is easy now, in case you want to flash down. Go to HTC - Developer Center, download Nexus One FRF91 system image and follow the instructions on http://forum.xda-dev...d.php?t=614850.

This post has been edited by Skipper: 17 August 2010 - 10:19 P"