I've never done much with electronics (while still trying to ensure that I know what I'm doing--I've definitely hooked random stuff up together before, lol), but I should definitely be able to do this. It looks like most case fans are from 10-20 mA. Are there really wall warts that don't support that much amperage?
edit: alright, first step down. Went to Good Will during lunch and found a single 12v ac adapter. Next step I'll go home and hope that my spare fans I've got laying around are 12v-it looks like most fans are, but I've never needed to pay attention to that part before and these are about 5 years old. Either way, fans are cheap enough that's not a big deal.
edit again: Hm, from looking at that diagram again, I see the circuit for the fan, but how does the thermometer have power to read the temperature without the fan being on? Does there not need to be one dedicated circuit for the thermometer and a conditional circuit for the fan? (this is probably just me not knowing enough about how the physical parts of technology work)