You can have your sexy app up in a couple of weeks. But maintaining it still takes time, no matter what language/framework you're using. Upgrading to a new version is in the end maintenance (security fixes, underlying framework improvements, and the refactorings that go hand in hand with upgrading).

If you go with rails you still can get to a point very quick, with the downside that you have to spend time for upgrading. But both the elegance and maturity of rails 3, in regard of it's age is quite unique. The older it gets the prettier.

[2]. Agreed. I've been working both on client projects and in startups. If you have 10 Rails projects from clients to support, this is a nightmare, (rails 1.2 - 3.0, from mongrels to passengers). It's hard to charge clients for a rails upgrade.

[3] I developed rails since 0.14 full-time. I think rails was/is very conservative regarding trendy things (except coffeescript).