It depends on your game. And yes you can zoom in and out, that works well.
On my game (www.arrowmania.com - see the videos), I designed it so I have a set of sprites (spritesheets) for retina and a set for normal res. My level maps are TMX tile maps, just one TMX file, I have tile PNG’s for both normal and retina though.
Arrow Mania runs on iPhone and iPad in one universal app. On the iPhone, most of the levels I have zoom out to 0.75f scale, these same levels are all non-zoomed (scale 1.0) on iPad. I also let the user pinch and zoom in/out a bit on both iPhone and iPad too. Obviously on the iPad you can see more of the map, since on the iPhone you are zoomed out to 0.75 you can see enough though.
And it runs on the 3G (slowest device I support) really well, 30+ fps. 3GS/iPad 1 and above 60 fps pretty much constantly.