In addition to developer previews of iOS 5 and Lion, Apple has seeded the developer preview of Safari 5.1 to Mac and Windows developers. According to Apple's Developer website, Safari 5.1 Developer Preview takes "advantage of new technologies, including support for full-screen webpages, media caching with the HTML5 application cache, and better graphics acceleration on Windows."
Yesterday Apple previewed some new features for Safari in Lion, including Reading List and Tap to Zoom. Devs can get their hands on these new features now with the latest build. The rest of us will have to wait until this July when Safari 5.1 will be released alongside Lion.