ASP can run on unix machine, but only with a special 'interpreter' engine like chilisoft. You're basically emulating ASP support, which is not always desirable, especially not in a production environment.
Most people find that PHP is easier to learn than ASP, and PHP is growing quickly, getting all sorts of cool functions compiled-in as standard. Things like database support for a trillion databases, the totally-cool GD library etc make it very easy to create very usefull scripts without the hassle of MS's COM shite (excuzes le mot)
If you're "bound" to use windows server, by all means use ASP, MS products work better on MS platforms.
But it you're free to use unix, PHP is probably the way to go.
As for .net, well, if that does catch on, how long do you think it will take the opensource community to create PHP support for it? π