Okay, attached is the simple theme I pulled together to hide the background box. This is what I'm using on my iPhone 4; I don't know if it will work on 3GS and below (if not, it's a simple fix; just replace my images with the right non-retina display versions). Here is what my lockscreen looks like with two themes running--a modified Typophone 4 and the Clear LockScreen one below:
If I wanted to, I could also just make one theme out of the two, moving the files from Clear LockScreen to the right spot in the Typophone folder. I may do that eventually just to keep things neat. Anyway, hope that helps!
(By the way, the Clear LockScreen folder also includes a clear bottom bar and slider background, and contains the lock slider seen in my image. If all you want is the top bar made clear, you can delete all the files except for the BarLCD ones.)