I would like to know if Ruby and IIS 7 can live together, using the same 80 port in my webserver.
Two disparate processes cannot share a port. This comes down to basics of socket programming. However, a sub-process can share a port with its parent, as seen with Apache on Windows (have a look using ProcessExplorer).
The best thing to do is to run IIS on port 80, and use it as a proxy to the rails server running on a separate port behind it.
You would use a host-mask filter to do this.
Or, you could use IIS to serve rails directly via FastCGI, though I would honestly use something that is more "normal" in the rails world, and simply proxy it through IIS.
We do something similar with a Apache TomCat install running a java site side by side with .NET sites.