I've just started with these boards but from what I can tell there isn't an easy way to "brick" them. Because of the architecture (Von Neueman vs Harvard) the method of loading and storing code is vastly different than other mcu's out there that you may have played with. It's easy to brick microcontrollers with bad fuse settings, uploading code that does bad things, or otherwise doing something stupid (I have several AVR's laying around that are "bricked" because of fuse settings).
Now letting the magic smoke out is something entirely different and is usually the result of pushing electrons where they shouldn't go (or pushing too many). In that case you do have to be VERY careful about anything external you hook up to the pi.
There are no buffers on the GPIO pins and they are wired directly to the chip. These guys are NOT 5v tolerant and if you try to drive them with 5v ttl you will most likely damage your chip. You MUST buy a level converter if you plan on interfacing with 5v.
Also, don't hook anything up do the NC pins on the GPIO header. What I did was buy a 40 pin ribbon cable, and a header shroud. I soldered the header shroud to a carrier board with header pins soldered to it. I then clipped all the pins marked NC so when I plug it into a breadboard (or what have you) I am assured that the NC pins are not connected.
Finally, NEVER, EVER, EVER, short the 3v to 5v on the GPIO header. That will most likely damage your chip (and it's a bad practice to short power supplies anyway
As long as you are careful and take proper precautions against accidents (move that can of soda off the workbench) you should be okay.
Definitely get that level converter though. Sparkfun and Adafruit both sell one.