Hey mrubenson. There's nothing there as OmniFocus currently exists, but here's a _very_ simple script (that you could create in Applescript Editor and save in ~/Library/Scripts/Applications/OmniFocus/) that, literally, tells you "Time is up" after two minutes.
Code:
delay 120
do shell script "say Time is up"
It's an easy way to do it, but useless if your sound isn't up! (If you have Growl installed, perhaps you could go that route, too.)