I'm not a Linkin Park fan, but I don't understand this reasoning. Hybrid Theory and Meteora were massive sellers. The safe "sell-out" option would have been to just make the same album again. They didn't though, since then they've made two very different albums and have taken artistic risks, and have not written simple middle of the road rock songs that would sell well.
That said, I think Minutes To Midnight and A Thousand Suns are awful ha. But still, I respect them for not making the same album over and over.
With all do respect, and I do mean that because I'm not here to spark a shit-war, just to discuss;
I consider MTM where they sold out. I say so simply because Nu-Metal was a dying genre when they went to record MTM. Limp Bizkit had flopped, System of a Down had broken up, and Staind..well, we all know what direction Staind took. So, I think they're more modern-rock/Nickelback sound was where they had hoped to gain sales. Just because they didn't get the sales, doesn't mean they didn't sell-out, because to me; they tried to get those sales.
Then again, this is just my opinion and we'll never really know because we weren't in their heads with them when they decided to write MTM.