It's interesting to see so many posters on a thread in Pedants' corner who clearly aren't pedants.
If they were pedants they'd know it's think.
I wouldn't be quite so emphatic, pigsDOfly, but I pretty much agree with you.
I was someone who said "thing" most of my life. When I was confronted with "think", I didn't insist I was right, I went away and looked for evidence. (I linked to some earlier in this thread, and even got corrected on part of what I said, which I welcome). It becomes clear that the theory that "think" came first (with the witty play on "think" being repeated) and "thing" being a mishearing is much more convincing than the alternatives.
Good pedantry weighs up what dictionaries say, examples from good writers, and the usage of competent speakers - it doesn't (just!) rely on one's own prejudice.