Marc is looking for the good way to implement an authentication layer for his Dancer app. Looks like the way to go is with a Plack::Middelware. Indeed, Franck Cuny explained us that middlewares are a really great way to address this kind of feature. That way the authentication layer is separated from the application. Thanks to his Dancer::Session::PSGI, this could be done really easily.
sukria's report from FPW 2010 mentions async support and more PSGI integration for Dancer.