I bought a LN52A550 from Ciruit City and it did the same thing after 3 hours. I could hear it turn on and the middle light come on, but no picture. I exchanged it the next day and no problems since.
We did notice that the exchanged model we got was packaged better and the promotional sticker on the left side was on it. The first did not have the sticker. We think they gave us a returned model.
When in doubt, make sure when you buy something new, it has a sticker!