Okay, basically, every comic book is selling fewer copies since 2008 began.
It's more or less across the board (if you take out crossover tie-ins, it's almost entirely across the board).
Therefore, if you wish to make a "Creator X's comics are not selling," then said argument would apply to pretty much every creator out there.
Brian Michael Bendis' Secret Invasion is doing great business at the moment, but his Ultimate Spider-Man title has lost 5,000 copies off of December 2007's sales (17,000 copies down from May 07 to May 08).
One of the top-selling books not tied into a crossover is Thor, which has dropped about 15,000 copies since December 2007.
It is still one of the top books on the market (#13, to be precise), it just is not selling as well.
Sales are simply down across the board.
So take that into context before making a "Creator X's comic is losing sales" argument - EVERYone is losing sales. It's not a big deal, I just hate when arguments are made out of context. "____'s sales are down! See, Creator X/Concept Y is a failure!" does not work when EVERYone (not directly tied to a crossover) is going down in sales.