In the US, it's certainly customary to tip the wait staff - presuming that there are wait staff!
At most buffets in the US there will still be someone that shows you to your table, brings you drinks, takes away your dirty plates, etc. In these cases most people would tip that person, normally by leaving some cash on the table when leaving. The amount varies greatly, but would normally be in range of $1-3 per person depending on the number of people, the price of the buffet, and the level of service provided.
For buffets where there is nobody who is acting as your wait staff (eg, in breakfast buffets in many cheaper hotels) then you would not normally be expected to leave a tip.
Buffets are certainly one of the grayer areas when it comes to tipping. Some people will tell you "yes", others "maybe", others "no" - and at the end of the day it's really up to you. As an Australian living in the US I frequently don't leave a tip for buffet-style meals depending on the level of service provided - whereas in a non-buffet restaurant I wouldn't consider not adding a ~15% tip to the bill.
In my experience, the US is fairly unique in this regard - I would not expect to tip staff at a buffet in any other country.