i updated to 4.3 while preserving my old baseband in hopes that one day there will be an unlock for the one i have. Jailbroke it, and good to go. Restored everything with pkgbackup, restore, and sync, and its exactly as it was before, just with 4.3.1
However, you gents are incorrect, if you updated to 4.3.1 using itunes, and apples stock firmware, and then jailbroke, your phone is jailbroken, but it is not unlocked. The only supported baseband for iphone 4 for a carrier unlock is 1.5.9, and if you updated from apples site, you don't have that anymore. So yes you are jailbroken, but no, you are not carrier unlocked. (unless you bought an unlocked iphone directly from apple in like hong kong, canada, or europe or something)
you can read ultrasn0w's info "only supports iphone 4 baseband 1.5.9" - unless you are using an older iphone with one of the other bbs listed