How can i use the 2x zoom function on my ipad for a iphone game, and let it show in retina rather then stretching the game? Thanks.
You can't. An iPhone game that doesn't support the iPad will play in normal 480x320 (pixel doubled in 2x). I doubt Apple will allow the iPad to play Retina-supporting games in full 960x640 because it could cannibalize iPad-specific HD version sales, which could piss off developers. Either wait for the devs to make their iPhone app universal or release an HD version specific to iPad.
Now that iPad is on iOS 4.x, the necessary methods should be baked in. I don't forsee it being a problem to add this to Retinasizer; in fact I was holding off on the update until now for that reason There is one catch: the iPad has a unique aspect ratio. So retina games can either be stretched (disproportionately) or the extra space can be empty (black).
I'm guessing/betting that jailbroken iPads can maybe already do that? I don't really know, because I don't have an iPad, nor have I ever used jailbreak. But seems like they have hacks for just about anything that Apple won't allow.
I would have thought that it would work exactly like the current 2x mode, but without the interpolation. In other words, black borders.
OK. I don't have an iPad (nor have I ever used one, except on jailbreakme day when I was busy JB'ing them in the Apple Store ) so I wasn't sure how it handled the size difference. I'll see what I can do (hard to test without an iPad though... one of these days I have to sit down and get mobilesubstrate compiled for the Simulator).
It doesn't work for all apps, but you could give Fullforce a try. It's free from Cydia. There's some more info here. http://www.simonblog.com/2010/05/29/fullforce-for-ipad-forces-iphone-app-to-display-in-full-screen-mode/
Preliminary tests have been successful. I can launch a game designed for 2x mode in 1x mode with 960x640 resolution. Release is imminent.