Basically you would take a NANDROID backup (which is a image of your device as it is) immediately after you root your phone. Then at any point in time you can revert back to this backup, and unroot your phone. Also: Clockwork Recovery will actually let you push an unrooted stock image of Android back onto your Droid phone.
As to bricking: you should be able to get yourself out of any tricky situation you might get yourself into. But if you follow all instructions, etc. you shouldn't have any problems. If you do have problems, try googling or referring to the XDA forums (the people there really focus on mods, hacking, etc. for Android phones and might be able to help you out). There are only a few ways to really brick your phone (installing the wrong cell radio for the phone would be one, for example).