As was said on here earlier in this thread, the problem with putting an i3 or i5 into the mini is the GPU, hence Apple would have to
1) Wait until intel release an IGP that is at least as fast as the 320m
OR
2) Go down the dedicated GPU route (which I would sincerely love them to do, but very much doubt they will).
I am in the same situation, I am waiting to see what the next revision of the mini brings. A mac pro would be overkill for my needs and I don't want the iMac unless they release a model with a matte screen.
I don't think putting a low end discrete GPU, such as an ATI Mobility Radeon 5470 (power consumption 13-15 watt) would be impossible to implement into the current form factor