What is it? The latest build of Apple's next generation desktop browser, based on their Open Source WebKit framework. Why do we care? Because the very same framework powers the desktop version's baby brother, MobileSafari on the iPhone, and while the 2.0 firmware brought some improvements to the latter, we ain't seen nothing like 4.0 yet.
Hot on the heels of the just released Safari 3.1, some of whose features are reportedly trickling down to baby brother MobileSafari on the iPhone 2.0 firmware, Apple has reportedly begun seeding early builds of Safari 4 (5526.11.2) to developers.