Actually, it's got over 80% support in almost all clients.
Gmail specifically do not support anything other than inline styles (no <style> or <link> tags allowed)
Lotus Notes only supports font related stuff.
Eudora has/had no CSS support at all (next version will be Thunderbird based)
But other than those and bloody Microsoft's crap, there's an average of 88% support of CSS in email clients:
AOL, Entourage, Hotmail, MacMail, New Yahoo, Old Yahoo, Outlook 2003, Thunderbird all have between 81% and 100% support.