Last week apple released new firmware update 3.1.3 for iPhone, this updates includes only few bugfixes and battery issues with 3GS. Dev-Team released their new version of Pwnage tool 3.1.5 for MAC which will let you do so safely, preserving your jailbreak and ultrasn0w unlock Yes you heard right only ultrasn0w users should try this.
Note:If you use the blacksn0w unlock (at baseband 05.11.07), you need to stay at 3.1.2 because blacksn0w is compatible with 5.11.07 but not 3.1.3 so until, geohot updates blacksn0w, it wont work.
iPhone 3GS users (regardless of unlock) should stay away from this and all 3.1.3 jailbreak tools unless you know you have your “SHSH hashes” backed up via Cydia. That’s because if you make a mistake you may find yourself stuck at official 3.1.3 with no way, to jailbreak or come back down to 3.1.2 to the jailbreak.
You can download PwnageTool for 3.1.5 for MAC fromhere (Official Torrent Link).
Please read Dev-Team’s blog for more details