mini (all) and first gen (core duo) imac will only support 2gb. owc, macworld, and a few others have published stories/tests of this after the new 3gb core 2 stuff came out. firmware limit it seems. so either apple fixes this or someone else 'fixes' the firmware... either way is fine with me 😉
edit: found my owc newsletter and will post this bit...
"One more quick note - 3GB of memory is achieved with one 2GB and one 1GB memory module. In our testing, didn't matter which slot which module was in + more importantly - installing 2 x 2GB still only gave you a total of 3GB usable although the 4GB were reported as being there. Further note that the original Intel iMac models remain limited to 2GB as do the current MacBook, MacBook Pro, and Mac mini Intel models. We have tested and found that a single 2GB module works in those machines, but the Apple hardware firmware seems to be what limits how much memory can be used. In the past it had been nice to take advantage of larger modules as they became available, I am not sure why Apple is putting the firmware limits on these Intel models. Perhaps there might one day be a hack the nips those limits, we will see."