We haven't tried this internally, however we are reasonably confident that it will not work out of the box. It would probably take an experienced person several days to get SilverStripe working reliable on IIS - and to date we haven't make this a priority.

If people think support for IIS should be made a priority, please let us know in this thread

I would like to see IIS support made a priority. IIS already works with PHP & MySQL. There shouldn't be much issue with allowing it to work on IIS as well.

That's not possible in IIS without addons.. but nowadays most of the search engines index also URLs with a "?" in it, so there should be a fallback-mechanism in SilverStripe.

I tried different things, but it's more complex than I initially thought because the controller has to sit in the root directory (otherwise all templates are broken because... inclusion of css and so on). But then we get errors with the ManifestBuilder.

I didn't looked in detail at the manifest builder yet, but I think it should be possible to fix it there. Then it would be possible to create a index.php that sets Director::setBaseURL('/index.php?url=') and it should work because we are in the right working directory.

I think this should be definitely a high priority feature for SilverStripe since approx. 30% of all servers on the net run on Microsoft servers(http://news.netcraft.com/archives/2007/04/02/april_2007_web_server_survey.html)

Maybe I will check this out later in summer, but in the near future I will focus on my main task - OpenID integration.

As far as I know there hasn't been any progress on this. You can try IIS Rewrite or ISAPI_Rewrite to achieve this.
There is also a lite version of ISAPI_Rewrite available which is completely free.

I hope this helps you!