When we last heard from Valve a year ago, Wine had about a .4% share of the steam user base. It would be interesting to see how this has changed since then - in that time, we've both released Wine 1.0 and likely seen Linux adoption increase a great deal. Linux had doubled its desktop marketshare the year preceding the last survey, I wouldn't be surprised to see Wine usage at around 1% on steam now.
Thanks, Scott Ritchie