If you're going to use the word artist, in order to have a really great definition, we should really examine that word, too. Is an artist someone who creates something original art? Because a session player, even if he's playing with heart & soul & spirit & technique etc, might be reading music someone else wrote, and would not be a musician under that definition of "artist."
I think we can look at this a different way and make progress faster, lol. Let's look at where the word "musician" comes from in the first place. The root word there is music, of course, which comes from the Latin word musica, which came from the earlier Greek word mousike, which comes from the even earlier Greek word, mousikos. Mousikos comes from compounding the suffix "ikos" (same as English -ic) to the root, proper noun Mousa (feminine form), translated to modern English as "muse." In other words, "music" originally meant art that was inspired by the Muses. In ancient Greece, people believed that the gods included 9 Muses, each in charge of a different type of performing art, and that these muses used humans as "mouths" for their creative expression ("muse" and "mouth" come from the same root word in ancient Greek, too).
Music is one of the nine types of performing arts the Muses brought to existence through humans; the others include epic poetry like Homer's The Iliad and The Odyssey, the telling of history (back then, history was more about teaching lessons than about a strict time-line of events, and so was much more artistic - the details were not the important part and varied from story-teller to story-teller, so long as the lessons were there), etc. Music as we think of it today was actually brought about by several muses, depending on what kind of music it was. (You can tell how important something is to a society, in a linguistic anthropology sense, by how many words they have in their lexicon to talk about something... just like all this discussion today about the definition of music!) The Greeks had a muse for sacred songs, another for song & dance (musicals), and another for music & lyric poetry (the way we think of "lyrics" today, even though technically, most contemporary songs don't use the "lyric" poetry form at all).
The Greeks believed that anyone, at any time, could be used as an instrument of the Muses, although some people were better "conduits" than others, but it all came from the gods, anyway. There wasn't any argument over who was a musician and who wasn't -- remember, this was also before recording technology existed, so music was not everywhere the way it is today. You couldn't go into a grocery store or a restaurant and hear music in the background; you couldn't flip on the radio while you were in your car, or listen to your iPod while you're walking across campus. If you wanted to hear music, you had to sing or play it yourself, or go see someone sing or play it live. Pretty much everyone sang and considered themselves as having the ability to sing, just like all children today learn songs like "The Itsy Bitsy Spider," etc, and don't think there is anything special about it - *everyone* was a "musician" back then; it was just accepted as music's place in society.
What I'm saying is, what "musician" means depends completely on context. It depends who you ask. In the United States in 2008, "musician," in my personal opinion, means someone who earns a living making music in some way - it's a job title. In ancient Greece, a "musician" was someone who happened to be inspired by the muses of music right then, lol. It depends on who you ask.
Art, though, as in creating something new and original from your own creative impulses, was not an accepted concept in ancient Greece. Anything creative was the gods speaking through you. So the question we must ask if we want to get to the bottom of this is, what do we want to consider "a musical artist"? It is someone trained in some instrument, or in music theory? Does it have to be original and creative and new? I don't think anyone would argue that professional concert pianists like Vladimir Horowitz are/were not musicians, even though Horowitz never wrote any music of his own (to my knowledge), and his technique, heart, soul etc were extraordinary.
This is an interesting topic. I'm curious to see where this leads!
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And yeah, I love that shirt, although I my profile pic (which is large) is a different pic for that exact reason, haha. That picture was taken on the same stage where I first saw Ani DiFranco live... that concert changed my life. When I played that same venue two years later, I wore the shirt I bought at her show, so I could stand where she was standing and get a picture of me wearing it there 
