Well there we go. Tried to install PHP into IIS7 as ISAPI using the windows installer from their web site. At first glance everything looks fine - but wait!
Running phpinfo() tells me it's running on FastCGI. What the ?!??
Some investigations later turns out that the PHP installation can't cope with IIS7 ISAPI, but Microsoft already includes a FastCGI PHP filter in IIS.
Curious as I am I tried to get it working on ISAPI adding a mapping filter to IIS. At first it all looks fine, but on closer inspection PHP refuses to load any extension modules. Odd cause reverting it back to FastCGI or CGI makes everything work perfectly. Even with a sginificant amount of tinkering around I just can't figure out where things go wrong. For the moment I've thus set everything to FastCGI - speed fortunately doesn't seem to be an issue.
If I find out more and get that to run one day - I'll update this post.