I leave my SS51 on 24/7 with a 2.26 P4, Radeon 9700 (as of last night, was an 8500 before), Western Digital 120gb 8 meg cache drive and a Plextor CD/DVD combo unit. Most of this time, it runs Distributed.net, thus the processor is pegged at 100%. Even on warm days (85-90 ambient tempeature inside) it works fine. (IE, CPU temp right at whatever I have the Fan gaurdian set to, 64 right now, and motherboard temps at 42). This could be due to the newer cooling solution in the 51 though, where the processor has a heatsync with heatpipes, and one 80mm fan with the radiator. The fan has a BIOS setting to specify when it should spin up higher, based off the internal CPU diode.
I have heard rumors of a SS50 -> SS51 upgrade, though I don't know the details.
Also, Shuttle will be releasing an AMD based box with the same cooling setup. It seems to work great, since the CPU heat is immediatly pushed out of the case for the most part.