We've had a few hours of downtime over the past year due to power failure. Our battery backups only last a few hours. Also, one of our databases became corrupt, but we backup the data faithfully so that only cost another hour or two.

Also, though I'd hesitate to call it downtime since the servers were up and running, but our T1 provider (Tier 1) has had their cable cut two different times. Meaning that even through everything was functioning as far as the databases were concerned, our clients were not able to access them. Furthermore, we were not able to send or receive many of the files necessary for DB updates. This cost us about 20 hours!