As a last update to the various Dev Centers that a few hours ago received new betas of iOS, OS X Lion and iCloud for developers, Apple has also posted the Safari 5.1 Developer Preview that, as seen on Lion (running 5.1 out of the box), will bring some of the browser's latest advancements to Snow Leopard. According to Apple:
You can now download Safari 5.1 Developer Preview and take advantage of new technologies, including support for full-screen webpages, media caching with the HTML5 application cache, and better graphics acceleration on Windows.
With Safari 5.1 Developer Preview, you can tap into new extension APIs, including support for popovers, menus, new event classes, and interaction with Reader.
The Developer Preview can be downloaded now from the Safari Dev Center.