No, seriously. David Brothers at 4thletter.net crunched the numbers (from Japan's best-selling manga in 2009 and a Bookscan analysis for 2008), and here's how it breaks down: in 2008, the entire number of comics sold, period, in the US, was 15,541,769, about half of which was just the top 750 titles. By contrast, One Piece sold 14,721,241 books in 2009-- in Japan alone.
However! It's important to note that we're comparing two different years; One Piece in Japan in '09 and all American comics in the US in '08. Additionally, the number for comics ONLY applies to stores that actually report their sales to Bookscan, so the number of comics sold is probably a bit higher.
But the biggest, most important factoid in here: as far as I can tell, the Bookscan list ONLY features collected graphic novels, not individual chapter floppies, which would doubtless add a lot of numbers to the US comics sales figure. (There's a similar offset back in One Piece's favor when you consider that its sales list is also only collected tankobon, and therefore does not include the number of copies of Weekly Shounen Jump, which features One Piece's chapters, sold in the year.)
(EDIT: So, WSJ has about a 215k circulation, adding about 11-12 million copies to One Piece's side, but individual chapters of comics sell about ~5-8 million overall per month, as far as I can tell, adding a minimum of 80 million copies to the US comics side-- so when you factor in chapters, One Piece still trails behind. But I still think this is all worth knowing!)
David Brothers' piece goes on to talk about why the series is such a hit (content-wise), which is an interesting read to anyone wanting to know more or wanting to brush up on their sales pitch for spreading the manga to others, so check it out if you're interested.
(VIa VIZ Media's Tweet.)