Damnation will surely not be a sellout album. By definition, a sellout album is one on which the artist or band compromises musical integrity in order to achieve a greater degree of commercial success. I think reading any Opeth interview shows how the band has high standards for its work and wouldn't take such a path. Damnation represents not a diluting of Opeth's style, but a cool change. I see no reason to fear the new album.
As for continuing with the direction of Damnation, there's no reason to think Opeth will get rid off all the trademarks (i.e. growls, guitars) and stick with the totally mellow theme. It would seem that Damnation is merely a departure for the here and now, an experimental album from a band that wanted to try something new.
I really can't wait to get my hands on it.