Alongside an updated set of developer tools, Microsoft is now making an early version of Windows Phone 7.5 available to developers to install on their unlocked retail devices today -- a crucial step in making sure studios have plenty of time to update their apps (and hopefully create new ones) to take advantage of Mango's many new features. Actually, the company is only encouraging its "most eager" devs to take a crack at installing the release right now while it irons out a "full distribution infrastructure" that'll be available at some point in the next couple weeks; in the meantime, Microsoft says that you should consider yourself a beta tester. Developers and beta testing usually go hand-in-hand, so I imagine that won't be discouraging most people.
Microsoft also notes that this is indeed an early, incomplete release (not far off from the build we reviewed, I suspect), so certain features will be missing. That said, up until now the only option devs have had for Mango is the software emulator -- nice to have, but no substitute for the real thing.