[Peter]

Its a Core 2 Duo Mac Mini running Leopard.
Everytime, from the second it powers up it runs the fans at 5200rpm (full) -- I've tried resetting the PRAM, resetting the power (where you plug it in with just power cord ...) and even limiting the fan speed with Fan Control -- the fan still runs at full.
Even if I limit the fan in Fan Control to 1200 rpm, it still runs it at 5200.
Any ideas?

[Simon]

Have you tried running Apple Hardware Test to see if maybe the fan or a sensor has gone bad?

[Peter]

no, I should probably do that.

[moonmonkey]

check the vents are not full of fluff, a pair of tweezers can clean it, no need to open it up.

[seanc]

Have you taken it apart recently? Sometimes there's a small cable that is forgotten about which will affect the fan.