In the past 2 years, I attended college courses in Physics 101 and 201 at UT Dallas, as well as a Princeton Review course for the MCAT's which include heavy study in basic physics, especially about the definitions of fundamental units and concepts.
As it turns out, Matt is correct in that the example of displacement returning to origin will result in an average velocity of 0. Both my physics professor as well as the Princeton Review instructor took special care to mention this specific example more than once because it is a common trick question on the MCATs. HOWEVER, and this is important. It is absolutely NOT the first and only example provided to explain the difference between velocity and speed. Nor did it come up during the first lecture about velocity. It was brought up long after we had repeatedly reviewed the concept, in an attempt to trip us up in order to demonstrate the exception. Defining velocity and then using zero displacement to explain it is similar to introducing the letter 'p' and using the word 'pneumonia' as your very first example. If you have a problem with that, take it up with the lexicographers and wordsmiths? -B
