I suspect what the dealer is offering is something called "Supaguard". They usually offer it in various [i]bundles[/i] e.g. exterior only for £250, exterior & interior + mats + a full tank of gas for £400, etc.
I went for the interior & exterior treatment as I'd had the exterior Supaguard done several years before on a new Nissan 200SX, and that car still looks the biz!
Would I have it done again? [b]No way![/b] Why not? Well, for one thing the car is no easier to clean than my previous IS which didn't have it. You still have to put just as much elbow grease into cleaning the outside of the car. The interior treatment was/is even worse. For one thing, every little bit of the dash that the mechanic missed with the treatment shows up slightly lighter. the other thing is dust - rather than reduce static and the amount of dust that clings to the dash, it makes it about ten times worse!
Seen comments about Supaguard before in these forums, and I think the concensus is that you can do it better/cheaper yourself.