I suppose that it depends on what you include in the category "metal".
The biggest selling "metal" CD so far has been VELVET REVOLVER's "CONTRABAND".
While I don't think it's really a metal album, "GET BORN" by JET has sold even more copies (probably going to be certified double platinum -- 2M units-- according to Soundscan, by December).
SHADOWS FALL's "WAR WITHIN" is probably going to be the biggest selling "true metal" album in the United States, but it will fall far short of going 'gold' (probably < 200K copies) in its first year of release.
Aside from about a half dozen other titles, it's been a terrible year for metal by almost any objective measure.
Commercial radio stations (most of which totally suck anyhow) are playing less metal. So are college-FM and other stations.
Internet radio is like a dumb joke: programmed BY losers, FOR losers -- a 100% monetary loss for the 'station owners' in most cases, with none even close to positive cash-flow.
Record companies are passing over more metal bands in favor of packaged acts (Ashlee Simpson, Nelly, Now That's What I call (crap|Music), Usher, Josh Groban, etc.).
All of the the
Billboard charts are dominated by Middle of the road, country and western, and aging stars, with hardly any metal at all.
Two U.S. metal tours have been cancelled, with the performers claiming that they were injured (that's to avoid getting sued by tour promoters and venue owners for breach of contracts -- really, the bands just can't afford the financial losses they'd incur by going on a U.S. tour this year). The cancelled tours mean insurance and other revenue losses. That will make promoters, venue owners, and others even more reluctant to book metal bands (especially on tours) next year.
In 2004, Metal is in rotten shape as a form of comercial music.
Personally, I blame the bands themselves, for turning away potential fans, and alienating investors and other backers. Most metal bands today pretty much kill themselves commercially with the image they project of themselves on their websites. They look repulsive; their attitudes and expectations are almost delusional (bands today have no 'business sense' at all); their music has no hooks to grab listeners; and even in concert, most bands fail to establish any rapport with their audiences.