Django or Ruby on Rails
funny... I'm in the same boat, though perhaps I've been looking around for a bit longer.
I came in from MS-land (C# too) and I went with Django, but only after I tinkered around with Google's AppEngine, which re-introduced me to Python.  Django is well organized, internally consistent (as far as I could tell), and well supported with a vibrant community.  Python is well organized, internally consistent (as far as I can tell), and extremely well supported with an amazing community.
Anyhow, there's more to rapid Python web development than Django.  You might want to search for: python wsgi framework.  Pylons and TurboGears offer enormous flexibility and focus on "best of breed" components, web2py appears to have a very cool DAL/ORM that I'm itching to dig into, and web.py and werkzeug (amongst others) are minimalistic and don't try to dictate how you should build your app.  And if you're sticking with Django, definitely check out the Pinax project - it might help kickstart your development efforts.
I'm a C#/.NET developer looking to mess around with something completely different - something LAM(*) stackish for building web apps quickly.
I'm thinking either Django or Rails. I kind of like the Python language better and it seems to be more full-featured than Ruby for statistical, scientific and networking (let me know if you think this is wrong). However, the RoR community seems to be much bigger - which might make it a safer bet. Do you think this is important? Anybody have experience with both RoR and Django?

Update: Specifically I'm looking to be able to build a community finance oriented site quickly. There are definitely a lot of Ruby Gems to speed up the process, but I noticed Pinax on the Django side which looks promising. 
I know that Python is already fairly popular for financial/mathematical programming. Anyone else have an opinion?

Update 2: Noticed some comments about ASP.NET MVC. I have in fact done pretty significant work with ASP.NET MVC - a LOB app for Medical Equipment Servicing, and I loved it. It is IMHO a much better and more intuitive way to write web apps compared with ASP.NET web forms. But, I'm really looking for a non-.NET development environment to develop some chops in.