Good news is this tweak has been updated to version 1.5 which comes with new real-time process feedback, new look, support for iPhone 4 Retina Display, New versioning and notification system. PopMark 1.5 is now compatible with iOS 4.2.1 firmware. You can watch the video embedded below to see it in action.
PopMark is available in Cydia under Big Boss repository. It costs $1.99 USD. In order to install this tweak you will need to have a jailbroken iPhone. If you don’t have jailbroken iPhone then you can follow the step by step instructions posted here to jailbreak your iPhone on iOS 4.2.1 with Redsn0w.