I'm currently planning out a web app that I want to host for people and allow them to host themselves on either Linux/Apache of IIS6 or IIS7 (for the benefits of bandwidth, directory services [login, etc.]). 
I see that PHP is supported on both platforms. I've heard people serving Django and Python in IIS using PyISAPIe. I'm not sure about Ruby/Rails on IIS until IronRuby ships. I don't have much Perl experience but understand it would run in IIS as well.
Anyone have input for me? Thanks in advance.
I have several production php5/6 applications that run on either windows/iis and apache/linux. switching between platforms has not been an issue for me. i test on a windows server talking to a mysql db on a linux machine. i deploy to a linux web server without issue. i cannot speak for rails or pytong as i'm not a ruby or python guy. however, they should work fine from what i understand of them. if i were you i'd pick the language you have the most experience with.
Compatibility with IIS and Apache -- PHP, Python, etc?