WinRumors is appropriately running a rumor story that states Microsoft is getting ready to send out the first-wave of Mango developer devices as early as next week. The article cites anonymously "sources familiar with the company’s plans" so take it with a grain of salt. Still, this is just a matter of when not if for Microsoft, so next week sounds as good as any other week (we also know that Microsoft is getting ready to show of the latest build and features soon, more on that later).
Previously, it was speculated that since most WP7 devs actually own Windows Phones now and they can sideload apps, Microsoft would opt to distribute early Mango builds via a ROM update. But between the number of devices, drivers and the problem of returning the device back to an initial state (if there's a problem) has made such a process implausible, not to mention logistically challenging. Instead, new dev devices with the OS pre-loaded look to go on loan to developers who want them.
Who gets them first, how to qualify and what type of devices is currently not known, but at least on the latter, modified HTC Trophys (with gyroscope), possibly the HTC Mazaa and something from Asus are possible.