What percentage of office workers (or "knowledge workers" if that's
easier) in the U.S. belong to Generation X?  What percentage are
Generation Y?

This is 1999 data:
baby boomers: 48%
generation x: 23%
generation y: 16%
(which leaves 13% for the veteran generation)
http://www.bls.gov/opub/mlr/2004/06/art2full.pdf

Obviously 6 years later, the veteran generation (and probably the baby
boomers) has lost part of its stake in the workforce and generation y
has gained some.  But unfortunately I couldn't find more recent data.

Thanks, that's close, but it refers to the entire workforce and
doesn't break the stats into work classifications (professional/white
collar, etc...).  I'm really looking for those generation stats
arrayed across job classifications.

Sorry, I did misread your question a bit.

That's really specific statistics you're looking for though, I'll be
quite suprised if anyone comes up with that.