First of all, in order to know if somebody has sold out, you will need to know their inner thoughts about why they have done what they have done. Therefore, labeling them a sell out is simply your judgement of what they have done now compared to what they have done in the past and your personal beliefs on what "sell out" is defined as.
Now let's take an individual who is only interested in money. They find that a certain genre of music is very popular right now, and they want some of the green flowing into that genre, so they pursue and are successful at said venture. Then a new genre of music comes along and they decide to switch gears into that genre, simply because there is more money in it. Are they a sell out? I think not as they went into this venture to make money and they are following the money. With Metallica, I have no clue about their story, but from what I have read in this thread, they were not focused on being the "Best metal band in the world" but instead being the "Biggest metal band in the world". Although this may not be "money" based (though one could conjecture that is what they meant when they said they wanted to be the biggest as in most sales), it is at the very least something other than an artistic pursuit. It is a goal of fans and adoring rather than making the best music you can possibly make.
Then of course there is the individual who is only interested in the art. They want to simply create the best music they can regardless of money returns or popularity. If that artist one day says "Fuck it, I am not going to listen to my inner self anymore and instead going to make whatever is going to make me the most money", that is a sell out in my definition. They have abandoned their original goal in pursuit of another goal. Now this new goal may have higher priority in their life now than the original goal did. When they were a teenager and life was easy and free, they could hold that artistic idealism while preaching from a pedestal, and then the Universe slapped them in the face as an adult. Rather than getting a job flipping burgers, they decided to make music that could make them money even though it was in conflict with the ideals that formed their initial musical ventures. So in such scenario, they are selling out on one level, but it is to pay for survival on another level (a level where they can still make music rather than doing a job they hate). This goes back to the idea of without knowing all their inner thoughts and a clear definition of the term, it is hard to say what a sell out really is.
I applaud anyone who is out there trying to make music in this day and age. As programs from schools continue to get cut, artists continue to get fucked with a spiked glove by corporate interests and the market is so saturated with content that not a single individual will ever hear even 1% of the music created in this world, it is a tough decision to devote your time and efforts to music. UNLESS YOU LOVE MAKING MUSIC! And if you do love making music, and you continue to do so regardless of the lack of returns (money, adoration, popularity), then you have not sold out in at least some sense of the phrase and are actually living the dream of any true artist, to create and deploy.
Lot's of good points made here and kudos to SoE for starting the thread! Good topic for debate ;~))