I rooted and unlocked my bootloader (and then rooted it again since unlocking the BL erases it) all this week.
When you unlock, everything that you installed gets wiped off... However anything on your internal SD and external SD still remains intact (yea, i was surprised when I went to my gallery folder and my pics were still there).
But that's why you have Titanium backup. Just restore all the stuff you installed before the unlock.
I however havent flashed a new ROM in yet; Still waiting for that Cyanogen Mod 7 to release. But I believe so since I've reflashed the original ROM again from backup b/c I was fiddling with the Webtop app and you can still restore your apps back. They're all saved on the internal/external storage.