My lab has a PS3 running Ubuntu Linux 9.04 Server Edition. After a period of a few hours with no use, the Ethernet connection (eth0) seems to go to sleep, causing the connection to be lost. Pinging or trying to SSH into the machine results in no response. The fix I've been using is to access the machine locally and restart it (trying to bring eth0 down then up doesn't seem to correct it).
I've tried setting up an hourly cron job that runs on the PS3 and pings another machine just to create network activity, but this doesn't seem to solve the problem either.
Update: The solution was to run the above cron job much more frequently: every 10 minutes works.