With the release of Magento Commerce Beta 1, I plan creating a Magento Demo Store that will run the Beta and will sell Magento Themes. The development team at Varien has not released the exact details on how Magento Themes will work, but they have stated that they are based on the Zend Framework.
The Magento Template structure has design packages that are easily uploadable in the Magento admin. Each Design Package has themes, with include layouts, templates, skins, and translations. The layouts are based on XML files that define the block structure. Templates are the HTML templates, and the skins are CSS and images. The translations have to do with the Zend_Translate class, which is part of the Zend Framework.
I'll post more once the Magento Forum and Wiki are opened to the public. The demo Magento Shop should be available in the next few days.
