On Reddit.com there's a recent post talking about an API-centric architecture that would separate out business logic from user logic.

I'm in a fairly unique position of being able to refactor an existing site that gets a moderate amount of traffic (~10k per day). Technically speaking, it probably does not NEED to be rebuilt from ground up, but as this is somewhat of a hobby project for me, I am choosing to rebuild it because I think there are significant long term benefits and because of the learning experience it affords me. [...] The solution that I've come up with, so far, is to create a single library whose sole responsibility is to handle system / business level interactions that occur. This API is completely segregated from any consumer facing frontend and de-coupled from any particular framework implementation.

Comments on the post are mostly supportive - they're in favor of the API-for-business-logic approach, but some recommend other methods than the single library attack. There's suggestions of using current frameworks to handle some of the "dirty work" involved in setting up the API and a mention of focusing on performance as well as functionality.