For those who might wonder about how open Microsoft is to third-party development tools and/or languages, here's one answer:
In response to feedback from Expression Web 1 users, we’ve added a rich set of features for PHP development in Expression Web version 2. There are the expected productivity features such as code-coloring, snippets and IntelliSense™, but it doesn’t stop there. The product now ships with a PHP development server that lets you test your PHP pages without any other external web server dependencies (e.g. IIS or Apache). Essentially, PHP is now a first-class citizen in Expression Web 2, and will continue to be in upcoming versions of the product.
For what it's worth, I'm not planning to start coding in PHP anytime soon, but for those folks who use it, whether regularly or occasionally, having another option for an editor, particularly one with great XHTML and CSS support too, can only be a good thing.