Single Page Applications are great if done right. They are highly responsive, easy to develop and feel fantastic. However, the initial loading state can be very painful sometimes. Twitter learned this and has moved away from their endless tornado of requests required before serving that initial page.
You can merge the two, but authenticating data was a sore point and I didn't see any good solutions out there. Through proxying and developing a good API with an authentication scheme modeled after GitHub's OAuth2 changes, I think I was pretty successful.
I wrote about my solution in detail at http://tech.tdp.me/2012/08/28/better-applications/