For now you can install jailbroken-apps via SSH - until we got the new Cydia version - as you have full root access on your iOS device.
STEP 1: Download redsn0w 0.9.6b2, iOS 4.2.1 GM and iOS 4.1 final version for iPhone 4, 3GS, 3G, iPod touch 4G, 3G (download links at the end).
STEP 2: Update to 4.2.1 GM firmware using the IPSW file you just downloaded via iTunes.
STEP 3: Launch Redsn0w 0.9.6b2, and Browse for iOS 4.1 firmware final version (NOT iOS 4.2.1 GM).
STEP 4: Select "Install Cydia" and click on Next
STEP 7: You will Get a Jailbroken Device on the latest iOS 4.2.1 GM as soon as the process ends.