Yet iago's request clearly showed "Server: Microsoft/IIS 7.0" or something like that.
You can usually disable that, too.
I'm not against security by obscurity, necessarily, but as long as you aren't relying on it, it doesn't hurt.
ASP.net is pretty damn obvious if you look at the source, though, unless you use all your own constructs and not their built-in fields, ajax, viewstate, etc etc.