Hi Stuart
It is an intriguing subject, and it does seem to make sense.
I cant remember where I found the article on how they have determined our Position and the Oscillations.
I do remember though, that Our solar system lies generaly above the Galactic plane and Oscillates up and down through the dense portion of the galactic plane with a specific period of around 26 million years to 35 million years For some reason or another there are two different estimates.
Check out this link at Wiki, it seems that a correlation between Extinction events and the Cycles are disputed and are not in sync with the Paleo record.
It only seems reasonable that since life has been here,and has survived previous oscillations from this Theory of cyclic impact events, the hypothesis is in question.
Here is the Link
http://en.wikipedia.org/wiki/Nemesis_%28star%29
Nevertheless