The canonical list of banjo jokes can be found at:
http://www.wsnet.com/~phil/banjokes.html

But in case the site is busy, here are my two favorites:

What's the quickest way to properly tune a banjo?
Wire cutters.

How do you tell different banjo tunes apart?
By their names.

Regarding viola players, what I've heard is that there are so MANY great violinists competing for places in symphonies these days, that some give up and take up a less competitive instrument, very similar to the violin -- the viola. Hence, viola players are often thought of as failed violinists.

How do you get three viola players to play in tune?
You shoot two of them.