So says THR's "Heat Vision" blog: http://www.heatvisionblog.com/2010/03/evans-accepts-captain-america-mantle.html
This all comes after Marvel Films essentially shortlisted, "considered" and screen-tested every "all-American"-looking male actor in the United States, up to and including Ryan Philippe, Channing Tatum and John Krasinski. If true, the deal not only gives Evans the lead in (at least) one major Marvel movie but also co-lead in "The Avengers" and (likely) cameos in "connected" Marvel movies. It also saddles him with the unenviable task of explaining to research-averse film journalists that it "doesn't matter" that he was already in the (terrible) "Fantastic Four" movies because the shared "Avengers Universe" movies Marvel is making doesn't (yet) include characters who's franchises are currently owned by other studios (re: Spider-Man, X-Men, Fanastic Four, etc.)
In the broader sense, this also turns the (still akwardly-titled) "The First Avenger: Captain America" into the capper on a pretty big - and interestingly comic-book-centric - year for Evans: He's also got big roles in the DC-based "The Losers" and Edgar Wright's "Scott Pilgrim" movie.
Additionally, for those who hadn't yet heard, the victor in the casting of The Red Skull - aka the battle of vocally-gifted pan-European character actors specializing in bad guys - ultimately went to Hugo Weaving.