+1 for RetinaPad. It does better at scaling up iPhone apps.
f4780y (another Hacking section moderator - our resident expert, actually!) posted this about FullForce (so you can better understand what FullForce does/doesn't do):
RetinaPad is exactly that you want, but I just wanted to clarify in defence of FullForce...
It fools applications which use standard iOS UI elements into using full screen iPad versions rather than being constrained into the iPhone screen size for which they were coded. That's why it only works on a limited number of apps, because not all apps use the standard IOS user interface elements, and games in particular will never work.
Just thought I would throw that in... FullForce is still a very good and free tool as long as you understand what it is being provided for! It does something quite different to RetinaPad.
And, to further my agreement with Grayson D), RetinaPad works great and is completely worth the price!
Because it only works for certain apps and apple doesnt want to have to deal with stupid people bothering them about it. And it also only scales up certain UI elements and not everything. So apple's "Make it work perfectly before release" concept applies.