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From the announcement:
Our first Drupal 7 alpha version was released just over a month ago. Today, we're proud to announce the release of the second alpha version of Drupal 7.x for your further testing and feedback. The first alpha announcement provided a comprehensive list of improvements made since Drupal 6.x, so in this announcement we'll concentrate on how you can help ensure that Drupal 7 is released as soon as possible and is as rock solid as the previous Drupal releases that you've grown to love!
The most notable change for developers is moving
$form_state. This makes field forms more resilient to form_alters and debugging became a lot less tedious. Comment body became a field and new fields can be added to comments through the field UI module. And congratulations to the docs team and associated helpers for their outstanding work in enhancing and correcting lots of API documentation this release.
We've also fixed a number of issues since the previous alpha, most importantly one that caused all files to be deleted after six hours.
You can download from the announcement or Drupal.org