Yeah, the MIDP has a way to get the phone resolution. All games I've seen though just have different versions for different phones, because you can't make many assumptions (if any) and developers try to keep the size at minimum (that many phones require the game to be downloaded to the internal memory instead of the SD card directly doesn't help either). Also performance reasons too, adapting code to specific hardware will be faster, and remember phones aren't exactly the fastest hardware available (some have no trouble and even can do 3D, but some struggle to just do 2D).