I'm thinking about moving a Wordpress install to some server running Server 2008 R2. It appears that Microsoft has been doing some to create running PHP more palatable, however i really haven't seen any recent data or real existence test cases.
I lately setup a wordpress install on the 2008 r2 server... Everything works fine aside from email... Wordpress is placed to transmit mail via sendmail/phpmail - so rather you have to allow it to be to ensure that wordpress transmits via SMTP. Apart from that - I'd no problems whatsoever :)
IIS does not have difficulties with threads by itself. IIS uses an application pool to process demands. Make certain you put in the NTS VC9 version of PHP and you're simply fine.
WordPress on IIS 7.5 is effective btw. Simplest method to do the installation is here now, http://web.ms/wordpress. You may either RDP to your server and install or do the installation for your local machine using WebMatrix, have it set up all in your area then deploy it for your server using WebMatrix's posting feature. This is particularly easy in case your host company supports Microsoft's WebDeploy.