Squarespace has just released their Developer Platform for Version 6 of their service. Version 6 launched a few weeks ago without the Developer Platform, to the dismay of many, with the promise that it would soon follow. That promise has now been made a reality.
What this means if you're a developer is that you can now build a custom template from the ground up. You will now have total control over every part of the site and can integrate it with all sorts of third party services to build extremely powerful CMSs.
What this means if you're not a developer is that you should start to see tons of third-party templates from all sorts of developers. You will no longer have to wait for Squarespace to release their own, official templates. There will soon (hopefully) be a vibrant community of third-party Squarespace templates — much like there is now for Wordpress, only smaller, no doubt.