How can I tell if my iOS device is currently jailbroken?
Basically, if you can execute arbitrary code or write to arbitrary disk locations, you're jailbroken.

iOS security researcher Stefan Esser (also knows as i0n1c) has developed an application called System and Security Info to determine if an iOS device is currently jailbroken by testing for the abilities that you gain from jailbreaking.

Update: Apple has taken down System and Security Info.