this concerns me also, as this is big reason why imacs break down within 2-3 years. with the poor ventilation and low default fan speeds, the imacs are like toasters. in early imac designs, the fan for the hdd would blow hot air right over the psu, which would cause it to overheat and shut down. i think they resolved this design flaw in recent imac models, but they still run too hot imo.
hopefully since the mini draws much less power than the imac, it won't have as many issues caused by heat. the new design supposedly draws in air through vents around the bottom and blows it out the back. but like the previous poster said, only time will tell if this design is viable.