Is ASP.NET or PHP better for building WEB 2.0 applications?
PHP is more universal since it can run on Windows, Linux, Unix, etc. ASP.net must run on a Windows web server (specifically IIS). You can use Mono on Linux and other platforms to develop and run .NET applications, but PHP is still more universal. You also have better chances of finding open source extensions and code for specific purposes for PHP. With ASP.net you will probably need to pay for some commercial components.