Lots of interesting interpretations around, many of which are mentioned above: Orson Scott Card, Terry Pratchett, Susan Cooper, etc.
The "7th son" is also mentioned a lot in the old fairy tales, such as Grimm's. Usually the young man succeeded in whatever quest he was on because his luck, magical powers, and/or charm were phenomenal (due to his being a 7th son, of course). Grimm's is German, but for some reason I always thought of the 7th son tradition being Celtic-based?
Sorry McPiper, but I don't recall anything that would imply that being a 7th son is less than wonderful. You may just have to gag your co-worker!