While I am certainly not a Microsoft bigot, there are a couple of factors that are hard to ignore when analyzing platforms:

First, while it is true that using Microsoft products can tend to marry you to a particular platform choice, in all practicality, any development team is ALWAYS married to one platform or another in the end no matter what is chosen. Even with PHP and MySQL, for serious applications accomodations are made in code based on the platform, machine architecture, etc.

In addition, for non-Microsoft based applications, there is very, very little truly integrated tool support. For a PHP based solution, there are no less than 3 different vendors involved (the PHP, MySQL, and Apache)–many of those open source vendors where there is no support if there is a bug. You are on your own to fix it. For pet projects, this is okay, but when there is real money involved, fiscal accountability is extremely important. It is possible to implement a Microsoft solution at low cost using only a single vendor accountable for all the parts and STILL support open standards that allow for integration with other applications on other platforms.

These days, you can get a dedicated IIS server for about the same cost as a Linux server through a hosting provider. These packages include support for SQL Server as well in many cases, and using Visual Studio Express Editions, which are FREE, development can be done at lower cost than a comparable PHP or Java solution, since the development time is so much shorter on Visual Studio, which integrates ALL the parts in a fluid way.

Just a few thoughts from the “other” side.