Because it's a CS5.5 file.
Thanks Bob, but sorry i might not explain myself very well. The original file is CS4 and indeed it did open just fine in CS5 until yesterday. Today, all of the sudden i got this message. I am not trying to open a CS5 into CS4...is the other way around and it worked before.
If you indeed have CS5 open that file was saved in CS5.5.