[player] is using modified data file.

[mortoopz 23]

What does this mean, and why is it cluttering up my screen all the time?... making me thing something important has happened.

[applejaxc 2500]

It means nothing, it got patched out. :)

[mercules 1290]

Left over from the engine. Basically there used to be a "Joined" announcement and then messages when someone was using a different data file than was "Official" to the server. It's left over from the ARMA stuff in the engine. 

You will be happy to know it is part of the next update to remove the message. 

[mortoopz 23]

I am indeed happy, that bugs me :)