Supposedly, according to one of my sources, Microsoft is going to make the Mango ROMs released to all regular developers (those who've paid the $99 fee to sign up). Regular developers will get Mango ROMs to flash on their registered, unlocked retail devices through the create.msdn.com portal, I hear.
If you're a developer who wants/needs access to next-generation hardware that Mango will support (with additions like gyroscopes), you'll have to contact your local evangelist and lobby to be one of the chosen few who will get loaner test devices.
When I asked a Microsoft spokesperson yesterday about its rumored beta build distribution plans, I received a no comment.
Microsoft has said to expect new phones running Mango to be available in time for this holiday season. The company hasn't shed any light on how long it might take for existing Windows Phone 7 customers to get the Mango update pushed to them, following carrier and handset testing.