I was working on J2ME games in the past, when draw a large backgroud, I always use the Carmack 2d scroll algorithm, keep a image buffer a little larger than device screen, draw the changed small tiles to this buffer, then draw the buffer to device screen. This can reduce the number of times to call drawImage().
Now we need to prepare for opengles 2D games for iphone and android. I am new about opengl, but i have try to read some engine code and keep learning opengl. There isn't much problem about 2d graphic system, except the tile buffer, there is no image buffer any more, so what size of the tile should i use? The screen size maybe 800*480, if i keep the 32*32 tile, i need to draw about 400 tiles per frame, will this be ok?