I've got a couple of different takes on this.
First, if I were new to writing web apps and wanted something I could get quickly proficient with, I'd probably pick ASP.NET because of how easy and powerful Visual Web Developer is. You can get a lot done with very little coding with that tool.
But second, as Old Pedant says, it's useful to be able to work with many languages. We just deployed these new forums using vBulletin which is written in PHP. Why? because vBulletin does so much of what we need already, the job was 90% or more done for us, we just did a little customization in PHP. In this case, we let the solution pick the language.
And I'll also throw out a few broad generalizations and I'll let the others here refute and poke holes in them:
- The .NET languages (ASP.NET, C#, and VB) have better penetration in enterprise use
- There's still a lot of Java programming done on big government jobs in the US
- I see PHP, python, and other open langauges getting their best footholds in newer startups, small non-profits, anywhere with fewer ties to enterprise buying habits
Google and Yahoo use a lot of PHP and Pythong I believe, both of them have also been at the forefront in JavaScript lately.
Learn as much as you can.