[bOR_]

Its tricky to answer, as I am limited to browsing wikipedia pages on these topics and no more knowledgeable than you are on the topic :-). One obvious alternative to your line of thinking is that the matter+anti-matter that is created in a vacuum is not 'normal' (using your position on normal matter), and has no mass.

One of the wikipedia pages seems to support that:

http://en.wikipedia.org/wiki/Virtual_particles "As such, virtual particles are also excitations of the underlying fields, but are detectable only as forces but not particles."

It seems to have something to do with normally existing only for a very short time, and affecting the universe only on a very short range that these virtual particles get away with appearing to not having a mass from the point of view of the rest of the universe. http://en.wikipedia.org/wiki/Virtual_particles