In a Channel 9 Live interview, Brandon Watson let it be known that Microsoft is working hard to enable an early release of Mango to developers.



From WP Central:

The obvious benefit is that this early release will allow developers to test the update before it reaches consumers. It will also give developers a jump on any updates and new software that will tap into any new features Mango will bring to Windows Phone 7.

  Developers may get early access to Mango