The reason u have ipad baseband 06.15.00 is cuz it was carrier unlocked, so if u updated to iOS 5.0.1 "stock" Apple firmware you've corrupted the baseband. U need to re-flash the iPad baseband & cook/create a custom firmware with redsn)w or sn)wbreeze. Simply put, do the following...
1. Re-flash iPad baseband.
2. Use sn0wbreeze to cook custom firmware. (make sure u answer "OLD or NEW" bootrom correctly)
3. use sn0wbreeze or redsn0w to enter "pwned' DFU, u need to enter "regular" DFU mode to get to "pwned" DFU mode.
3. Restore via iTunes by holding "SHIFT' on ur keyboard while clicking "Restore" & select the custom firmware that has no stock/lower baseband therefore not corrupting the iPad/higher basebamd that's needed for your unlock. If unlock is not needed, u still must always use custom/cooked .ipsw/iOS firmware cuz of the 06.15.00 iPad baseband that can never be downgraded.
If ur old bootrom this will be easy without shsh blobs & TinyUmbrella. Or if it's new bootrom & ur below week 135 in ur serial #'2 3rd, 4th & 5th digits, it'll also be ok but if ur new bootroom & newer than week 135 in ur serial number u may be S.O.L.!
Good luck, I'll check back later & if I'm incorrect anyone please chime in. Or if I think of anything else or u give update I'll help k.