As usual, the details of this release remain behind the NDA wall, so you'll need to visit the developer site and check them out yourself. Apple will be releasing 4.0 into the general iPhone population this summer. An iPad version will debut in the fall.
Apple's servers get slammed whenever a new SDK is released. You may want to wait a few hours if you encounter any problems connecting to the developer site.
UPDATE: Apple has pulled the beta for now. This has happened before and generally indicates some problem with the build (despite paranoia about leaked features). Please let us know when Beta 3 returns.
UPDATE 2: The beta is available for download once again. The beta returned around 9:30 PM (Pacific Time). Given that the beta was only pulled for less than one day, and judging from comments made by developers who managed to grab the beta this morning, chances are the reason for the beta being pulled may have been due to installation location problems. Now that the download is back, we'll find out soon enough.