A new MobileInstallation patch is required for iPhone users who updated to 3.0. While some people report that you can use the MobileInstallation patch for 3.0 beta, there is also another solution.
Simply go to Cydia and add the following source: http://cydia.hackulo.us.
Then install the application called AppSync. This will patch your MobileInstallation file.
You can find all MobileInstallation patch files for all firmwares here.
