Thou shalt not take the Lord's name in vain?

"Thou shalt not take the name of the LORD thy God in vain; for the LORD will not hold him guiltless that taketh his name in vain." -Exodus 20:7

I saw a billboard while driving through/past Des Moines or Ankeny (they're all the same in my mind), which read "If you're going to swear, use your own name -God" Or something to the effect. I'm sure that whoever put up that builboard meant well, but it would appear to be in itself a violation of the 3rd commandment. (it is actually the third, right? God, idol, name, day, honor, murder, adultery, steal, lie, covet...yep) So hmm. I wish I knew what "in vain" meant exactly, or a little more specifically anyway. It was definitely using God's name in a frivolous and therefore unseemly manner.