While you hopefully don't need one-button access to 911, a desk-mounted panic button is undoubtedly cool and it makes for a great DIY Arduino project. You'll need a prepaid SIM card, an Arduino, a GPRS shield and a few other odds and ends.
Luckily, Instructables user randofo created a very thorough guide to help you out. You'll need to be comfortable with soldering, but if you can get it done, you'll have one button that can call any phone in the world. Randofo uses it to create fake calls at his desk to get rid of annoying co-workers, but I'm sure you can think of some other creative uses.
Desk Panic Button [Instructables]