A mac mini would require the OP to buy a monitor, no? Unless they have one. White MB is definitely a good way to go, or maybe even look into getting a refurb model or a MB or MBP, since you only seem to want it to make apps.
I'm not sure what to do. I want to buy a mac so I can develop iPhone apps..
Refurbished Mac Mini plus a large (in terms of pixels) monitor. You don't need the performance but will find lots of pixels are convenient.
You could also seek out a used mini or iMac. Any model with a Core 2 Duo would be fine.
I would disagree with that statement... my game now takes about 4-5 minutes to compile from a fresh clean on my old core 2 duo 27" iMac (just sold it to buy a 27" i7)
But wouldn't you mostly be incrementally recompiling changes? I don't do Mac/iPhone programming but I assume XCode will do this. I did a similar upgrade, but only for faster image and video processing and the high res display. Compilation speeds (Java, C cross-compilation, some Visual Studio under Parallels) was essentially instantaneous even with the C2D or my Pentium 4 PC at work.