Many times, people just mount the MOBO to the inside of the cabinet or to a draw that can be removed or pulled out.
Probably just as often though, people just stick the whole PC in there, case and all. You might lash it down with zip ties or something but it's really no big deal either way.
The question is how much do you think you'll be working on it, replacing elements, upgrading etc. Go for whats easiest to maintain.
Now, if youre cabinet is cramped for space, say a cocktail or similar, you may have to get creative, but for a full size cab, you have lots of room to play with.
For pics, I'd probably start with the hall of fame worthy projects
http://forum.arcadecontrols.com/index.php/topic,103103.0.html