Ok this is what I just did to get go from rooted froyo to unrooted S-ON froyo. The S-ON was the most time consuming part.
1-
download and place into the root of you micro SD card reboot into hboot and and volume up to run update.
2- Root again so you can
S-On. This is needed so you can use the Clockworkmod recover in the next step.
3- Rename PB31IMG.zip that you downloaded in step one to update.zip and place in the root of your micro SD card. And reboot into hboot again.
4- Now in 'hboot' select 'recovery', goto 'run update.zip' and select and run. Now you are S-On and no longer have clockworkmod installed.
5- Download
this, rename to 'update.zip'.
5a- Reboot into hboot, select recovery. Now the screen will have a funny picture, pretty the volume up key and power button. Now select 'run update.zip'. This will put you at 2.1 update 1 which is where you want to be to upgrade to Froyo.
6- Download and rename
this to update.zip. Follow step 5a, and you are done.
All of this info was found on the xda-developer site, and of course the fantastic Android central site!
Matt