Upgraded to CVS snapshot: can I go back?
Upgraded my pfsense-1.0.1-updated to last CVS snapshot.
Would like to revert back to my old pfsense-1.0.1-updated.
Is that possible from shell or webgui?
No. Upload 1.0.1 firmware to go back.
do you mean go under "system" -> "firmware" and follow the update procedure from there?
Is it ok to use the pfSense-Full-Update-1.0.1.tgz, or should I use another one?
Yes and yes.