Technically it's against the ToS to edit .dats but it depends really on what you're doing it for.
The main concerns with .dat edits is changing things like coffers to giant pink flamingos making them more noticable and such, or removing .dats of mobs so that you don't see them (i.e. Darters in Dragon's Aery)
The general concensus though is that if you're doing it just for aesthetics it's ok and there are a number of sites that have .dats availabe to use and even overhaul the UI.
http://www.ffximc.com/index.php http://brandson.livejournal.com/ http://www.nomido.at/
Those are just a few sites that have .dats and tutorials on .dats. The Brandson one is a UI edit that I personally like.
To my knowledge .dat swaps are not detectable as they only changes things client-side and will be removed if you run a file check reinstalling the original .dats.