Well I can't speak for everyone but when I use the term sell out in reference to music or I hear the term I associate it with a band that "tones down" (anything from a metal band playing less hard music or writing less controversial lyrics, etc) their music so it can get more mainstream radio airtime or the music sounds almost like a commerical jingle that everyone can sing along to. The chorus in "What I've done" fits this perfectly. The song is basically two big chorus in fact. I never liked U2 but they are some of the biggest sell outs. That song Vertigo fits the commerical jingle pattern perfectly.

Linkin Park is now on regular rotation on stations that play things like soft rock and top 40.

It doesn't bother me so much that Linkin Park made some songs that are more mainstream and get more airtime but what does bother me is that the songs suck.