There will probably always be more .NET jobs than PHP/MySql jobs. The reason being (in my opinion), is the proprietary .NET architecture and the knowledge needed to tie in multiple technologies (VB.NET, MS SQL Server, etc.)

PHP/MySql isn't as complicated (or restrictive), and programmers may find it easier to learn. Plus, some will say it's more efficient (running on Apache) than the ASP.NET platform.

Whether or not it's worth learning is difficult to say. I don't think .NET is going anywhere and it's always good to be diverse.

You'll probably want to look at Microsoft Certified Solution Developer (MCSD) for Microsoft .NET certification. You should be able to take the new Microsoft Certified Technology Specialist exams for Microsoft SQL Server 2005 and Microsoft BizTalk Server 2006 as electives towards getting your MCSD.

Coming from a PHP background, I just recently got promoted at my current job and one of my new duties is to train myself in .NET. I can tell you that I wish I was being sent to the certification classes as it's quite difficult to acquire a working knowledge of it on my own.

Good luck if you decide to do it.