No, you aren't. The Dev-Team is working on a multi-platform (Mac/Win/Lin) version of PwnageTool. You can also obtain a custom IPSW file from the internet. Please don't ask here; we are not allowed to distribute copyrighted materials.
Finally, you could always update to 3.1.2 and then jailbreak with blackra1n and unlock with blacksn0w. Be aware that updating to 3.1.2 stock will upgrade your baseband and you will no longer be able to use ultrasn0w. Also be aware that a minority of blacksn0w users have experienced intermittent WiFi connectivity problems with blacksn0w.