- MultiAlarm -- free, open source
- JackSMS -- free
- iAlertU -- donationware
- Theft Sensor -- public beta
[robg adds: I haven't tested any of these, but they do look somewhat useful -- though only if you leave your laptop on and not sleeping when you step away. Personally, I'll never step away from my machine, even for a second, in a public place. It's just too risky...]

