I see pretty comparable numbers of listings for Python, Ruby/RoR, Java, and .Net (although if you add up all of the nix web dev jobs and bundle Python, PHP, and Ruby together vs. .Net, you're going to see a lot more of the former).
For web dev in general the split is pretty even, but if I am looking more at startups or small shops, the balance shifts towards Python/RoR (with Javascript being pretty ubiquitous whether you are on .Net, Java, RoR, or Python).
Still see this more of a tooling cost issue though, which makes sense. MS licensing is not cheap, although I've worked with a suprising number of startups that are on the MS stack, or using a combination of languages.
Do you think all those ASP.Net sites are more likely to be delivered on the web to a diverse selection of evolving, modern user agents, or on an internal network to only IE7 users until 2019?
One opportunity has a choice in development tools. The other doesn't, really. The former market for developer mindshare can and does change; the latter is Microsoft's entrenched base which is never really in jeopardy (nor very relevant to the growth of the tech field at large).
