I do ASP.NET professionally, and PHP and Perl for side projects. Earlier posts were right on...ASP.NET scales much better than PHP or classic ASP does, but it costs more to host on account if it needing to run on a windows server. Microsoft has free IDEs for beginning ASP.NET developers...You don't need to pony up the $500 for Visual Studio.I think ASP.NET is favored by large organizations because the extra money they spend on infrastructure is nothing compared to how much they save in developer costs because ASP.NET is much, much faster to develop large applications in than PHP.
PHP is definitely more clean and concise than classic ASP (with vbscript or javascript), but it's a mess compared to C#, J#, or even VB.NET, which are the common languages used for ASP.NET apps.
For quick and dirty sites that don't need a ton of code, PHP is the better choice (which is why it's so popular...most sites don't need ASP.NET scalability.) I love PHP and am actually almost into beta testing with a PHP IDE I've been working on for the last year or so (ironically written using .NET). I'm trying to solve some of the scalability problems that make it inferior to ASP.NET for large projects.
It's also worth noting that professionally, the job market is better in quantity and dollars for ASP.NET developers. The best offer I've ever received doing PHP was under $40k per year, but it's not too tough to get into the six figures as a .NET developer.
I haven't looked into ASP.NET 3 (since 2 is only a year old), but I doubt it will be leaps and bounds over v2.0.
Good luck!
-SN