I posted some info about this in another thread ('Marvel is not necessarily doing better than DC'

. Here it is:
Checking Diamond's own site I found the following: From the page
http://www.diamondcomics.com/market_share.html -
'The Actual Dollar Market Share chart tracks all comics and magazine sales, including sales from initial orders, advance reorders, and reorders. November comics, graphic novels and magazine publisher actual dollar market share.
DC Comics 33% Marvel Comics 31.01% '
DC dollar values are almost 2% higher once reorders are factored in.
At Comicbookresources I found the following info. These figures were supplied to them by Diamond. From the page
http://www.comicbookresources.com/news/newsitem.cgi?
id=1763 -
'Actual Sales Based on All Orders and Reorders for Products Invoiced in November, 2002
Comics, Mgzns. & Graphic Novels dollar share DC COMICS 31.82% MARVEL COMICS 30.93%
Comics, Mgzns. & Graphic Novels unit share DC COMICS 33.18% MARVEL COMICS 38.46%
Hope this helps!