I saw this joke online today and I got it right away and thought it was quite humorous:

Q: Why do computer geeks confuse Halloween & Christmas?

A: 31 OCT = 25 DEC

So I rushed off to tell my fellow programmers and they just didn’t get it. I even wrote it down for them. Hmph. A joke is just not funny when you have to explain so I won’t.

Along the same lines I also like the often quoted:

There are only 10 types of people in the world… Those who understand binary and those who don’t.