An interesting second-source is Port80 Software's
Top 1000 Web Servers survey. This survey scans the web servers of the US Fortune 1000 companies - at the last count, back in June, 53.9% of these sites ran some version of IIS. I'm concerned
that 5.1% still run IIS 4.0 on NT 4.0 on the public Internet!
ISPs and blogs skew the figures heavily - both
my ISP site and my blog site are hosted, and both have their own domain names. The blog runs Apache on Linux (it's BlogSpot) and the ISP site runs something called thttpd/1.00.disbu (Demon Internet Small Business
Unit, maybe?) on Solaris 8. If I were self-hosting, I'd go with IIS 6.0 - I know it well, it's not had many vulnerabilities, and I'd want to use ASP.NET.
But I don't think IIS and Apache are really competing in the same market. I
wrote about this last December on my blog.