The iPhone DevTeam has just released jailbreak utility PwnageTool 4.1 for Mac OS X.
PwnageTool allows you to restore to a custom IPSW file. The main advantage of PwnageTool is for unlockers. It gives the ability to keep current baseband and preserve ultrasn0w unlock. This is only for those, who have not updated to iOS 4.1 by themseleves.
You can also add whatever packages you want in the “Expert” mode of PwnageTool, if you wish to pre-install something from Cydia.
Here is a list of supported devices:
You can download PwnageTool 4.1 here.