Can you give more details about the app? It might not seem relevant, but it often is. For instance, if you're just building a simple CMS with some custom bells&whistles wordpress would probably be the way to go. It had a pretty good plugin architecture, plenty of docs & support, and powers a good number of sites with ease.
However, if its a little more custom, I'd say for very rapid application building you can't go too wrong with Ruby on Rails. Personally I'm (very) not a fan, but there are lots of tutorials & forums to get help with both the language and framework. Ruby Gems are a nice touch, too. Saying that, I wouldn't choose it for anything which is going to get a lot of traffic, at least not until maglev is out.
Another problem with Ruby is that you'd have to learn a new language... not good if you just want productivity. You can get things done quickly in PHP but its often messy... you could maybe try groovy if you're happy to run on the java vm... again, its all based around how much you're willing to move from your comfort zone and what the project entails.