PHIL - What exactly draws the line at taking the Lord's Name in Vain? I'm aware that there are the obvious ones, such as "Oh my G*d!" and putting "Christ" or "Jesus Christ" in certain sentences. But I've always wondered about saying "Thank God." Is that really considered taking his name in Vain? It seems to me like almost a good thing to say. Any thoughts on this?

JOEL - It all depends on the context of what you are saying when using the Lord's name. Any time it is used out of anger or in describing something unrelated to diety it would be considered inappropriate. If you are actually respectfully talking about God or addressing Him as if in prayer, the use of His name would be appropriate. If you truly mean to thank God when you say "thank God" then, in my opinion, I see nothing wrong with it. I have heard General Authorities use that expression occasionally.
Sometimes even the expression "Oh my G*d" or something similar, might be an understandible thing one might say if they are under great mental, emotional, or physical distress and are truly addressing God as if begging for His help. But for the most part the expression has become all to common in normal discussion and would therefore qualify as taking His name in vain.
We have also been counseled to not use the sacred name of God too often in our speech or prayers. According to Elder George Q. Cannon:

"when you pray, address Him as Father, and when you speak of Him, do so as our Heavenly Father, or Lord, and do not use His sacred name lightly or frequently, neither in praying nor speaking; and remember that He "will not hold him guiltless that taketh his name in vain.(Exodus 20:7)" (Sept. 1, 1877)

So it might be inappropriate for a person to habitually use the expression "thank God", if they are not really thinking about what they are saying.
I think it is fairly easy to tell when someone is using the name of the Lord in vain. The place to "draw the line" for the few other uncertain situations can only be a matter of opinion. And much of what I have said here is of course my opinion.