If your T-Mobile's G1 Android firmware is version 1.0 TC4-RC29 or earlier, you have an unintended extra "feature" - open the slider, press 'Enter' to pull up the application tab, then type in "reboot" and press 'Enter' again. Your G1 will follow your command and reboot your phone.
This bug causes Android to interpret words as commands and executes them as "superuser privileges." It is not yet known which other words can be interpreted as commands, but it's best to be wary of this bug and eliminate it via the latest firmware upgrade. If you are like me and have an unlocked G1, you will have to update your firmware manually when RC30 is available (Stay tuned! We'll post it when we get it!). Until then, be careful what you type!