Been done before 
Oh, and that last machine I posted in that thread?
5:15PM up 612 days, 19:46, 0 users, load averages: 0.08, 0.02, 0.01
FreeBSD webserver, P166, 64MB, 2.5GB HDD. And it was active up until a week ago. Now she sits around while we wait to celebrate.
Seriously now, I've seen a couple of (FreeBSD primarily) systems with over 3 years... and there are doubtless many systems around academia that are still running, long since forgotten.