The Springboard blog included this chart to explain the differences between Mainstream and Extended support:
As far as we know, Microsoft has no plans to release a second service pack for Windows 7. Many of us Microsoft watchers have asked the company about this but received no comment. Microsoft is believed to be moving to a new model, via which it delivers annual refreshes of Windows, rather than big-bang releases once every three years or so, with service packs delivered in the interim.
The new model will likely change policies for some IT shops that have waited until Microsoft ships SP1 of a new version of Windows before even considering to move to the latest release.
The Office division at Microsoft seems to be on a path of sticking with the Service Pack model for the foreseeable future.