Back at MIX11, Joe Belfiore tweeted that the Windows Phone team was working on a way to get 'Mango' into developer hands well before fall (when released generally). The makes sense as it gives devs a new chance to try all the new features and get their software checked out on an actual device.
Speculation was that Microsoft would provide ROM updates to devs at some point during the summer, but alas that seems to not be the case. On the AppHub page (where devs discuss the platform), a section has been updated stating that Microsoft will be once again using new hardware to distribute the new OS:
We're working on a plan. Just like last year, when we made developer phones available with the Windows Phone 7 OS, we plan to have developer phones this summer supporting new Mango features like the gyroscope. We don't have any dates to share just yet, so stay tuned.
Considering there are many current devs with Windows Phones, it seems odd at first that it won't be a simple ROM flash. But considering the likely hood of people "ripping" the ROM and distributing it early (and we know that would happen), on top of the logistics of sending out single ROM updates for 10 devices and we can see why Microsoft would do the hardware route.
Still, the question arises of who qualifies, how many devices will there be, for how long do you have them, etc. For that, we'll just have to do what Microsoft says and stay tuned.