Thieves are not known for keeping their hands to themselves. The steal skill is pretty much proof of that. Stealing items from others is made possible through the steal skill itself, provided that equipment is not currently equipped. Then again, if your target isn't conscious... well, that's another story. Keep in mind that stealing while in combat is far too difficult a task, even for the greatest of thieves, and that others most likely will not appreciate you liberating their items. If they catch you, it might be time to run.