I like starting web apps with web.py, and I like building what may end up being merely demoware or a narrow tech prototype using web.py. I think it excels at whipping up a dynamic Hello World web app in just a few lines of code, with no boilerplate directory tree or scriptage, and no messing around with external servers, and no external code dependencies. Add sqlite when need persistence. Add memcached when need caching. Grow and refactor as you go.

But if I'm very confident it's going to become part of a long-lived dynamic web service with enough of the features Django provides anyway -- especially that awesome built-in admin UI -- then yes I start with Django. The mindshare/community factor is helpful too. I know there are other web frameworks which are decent but going with a big framework other than Django in the Python space today is a bit like going with FreeBSD rather than Linux when all you really need is some sort of free Unix. To pick Linux is to get a bigger ecosystem today and therefore probably also a longer supported future.