iPhonBuzz: Messages tweak allows users who have a jailbroken iOS device to compose SMS or respond to messages within any iOS screen thanks to the clever use of gesture.
Save yourself the 3.99 and get the free version of BiteSMS. It's flawless and serves the same functionality.
Came to the comments to say exactly this. BiteSMS has been giving jailbroken users the same functionality (but with a lot more features) for years.
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