I have a dev model which I received in June 2009. It ran 24x7 (with the exception of one week during which it was moved to a new location 2,500 miles away) until today.
The PSU died. I wired up a 5V 2.4A power supply from a USB hub I had lying around, it's working fine again. I've been tarring (with bzip2) 30GB of data on it for the last 8 hours or so; it's about as loaded as it will get and is going strong. The wall wart is only slightly warm to the touch.
I guess, from reading other posts, that my PSU lasted about a year longer than it should have
PS: The silver box next to the drobo is a Neoware thin client (800MHz Via) running pfsense, the best OSS firewall ever.