I’m now running WordPress on SQL 2008 R2 SQL Express, no issues with posting and related items, haven’t loaded many plugins yet so it will be a while before I can tell you that everything works well. It seems to be a decent setup, you install a must-use (MU) plugin and aside from putting a file called fields_map.parsed_types.php in the wp-content folder it’s all in once place.
If you want to give it a try, this page is definitely the source. Good instructions, I was able to get it running without much problem on IIS 7 and then got stuck with some kind of permission problem where I couldn’t get it working from the outside network. I suspect that was lack of IIS 7 experience or some server weirdness, because the initial set up went fine. I used the MS Web Installer for PHP, the SQL driver, and some related items, and then downloaded the SQL specific WP distribution. Finally I gave up on the IIS 7 plan and put it on a different server with IIS 5 and that worked fine, even with pointing it back to the first server that had the data. I don’t know that my experience is typical – if you look at the directions it’s a straight forward list of steps.
It’s a relief to be back on SQL Server, back in my comfort zone. Nothing wrong with MySQL for blog hosting (or many other things),but the combination of less mature tools and me having more experience with SQL just make it a much better solution for me.