RIM does a fine job of burying the "wipe handheld" function on its devices so you don't have to worry about every accidentally stumbling upon the feature. But, if you're getting rid of a device, especially if you're selling it, you want it to be clean.

To reset the BlackBerry Pearl 8110, for instance, go to the home screen then select Options-->Security Options-->General Settings and then press the Menu button. Wipe Handheld comes up in the menu, though it is not listed on the screen.

Wiping the handheld will reset it to the default settings it shipped with—clearing all user files and additional applications. It should not affect data stored on a separate microSD card, but you might want to remove your card just for peace of mind.