Don't Do It
DON'T DO IT YET. you sound like me. everything customized and mms-lovely, but I like to be updated ... anyway, some one or combination of my beloved cydia apps kept my phone from booting after restoring from iTunes successfully, and then APT-restoring those Cydia apps. 5 hours it took me trying to get it to reboot, then frustratedly to get back to 3.0!
Note that iTunes 9 will move all your old ipsws to the trash, if they are in user/Library/iTunes/iPhone software updates. So you have to fish your custom 3.0 out of there too. but major prob was getting phone back into restore mode when it was stuck.
From now on, I'm keeping my eye on the "Recommended firmware" version in small type at the top of the Bigboss.org page before updating.