While I'm very happy to see this, this leads into one of my sore points about this platform I use and love, so if you'll forgive a bit of a rant...
Frankly, it's about time. Seeing that others have "hacked" it onto WM6 and 6.1 devices within days of Marketplace going "live" tells us it wasn't a compatibility issue. If not compatibility, then what?
I understand the complex relationship between Microsoft and the OEMs makes it impossible for Microsoft to release "generic" OS updates for everyone, and because OEMs would much rather sell new devices to all of us than upgrade our old ones, getting OS updates on older hardware, even hardware fully capable of running the new OS won't be possible. Even so, sometimes I feel like MS compounds the problem everytime a new OS is released by treating older device owners like second-class citizens. Sure, I realize the new OS has some features not present in prior releases, but they often add insult to injury with their own applications, like Marketplace.
Every now and then I pull out one of my old WM5, or even WM2003 devices and say to myself, "this isn't all that different from 6.x- why doesn't Live Mesh, or MyPhone, or Facebook, or whatever run on it?" Is there really some magic in the WM6 SDK that prevents prior devices from utilizing what seem like fairly simple programs (from the device's end, I mean- I realize there's a lot going on "behind the curtain" with cloud services like Mesh or MyPhone)? Heck, Bing Mobile even runs on WM2003 devices, and frankly that seems just as complex (actually more so) as any of MS' recent WM apps. Or is it that adding compatibility for the last-gen OS just isn't worth the effort?
While there probably aren't that many people still using older devices, the advantage of supporting prior generations of OS and hardware isn't just for those few people still clinging to them for whatever reason- it sends a message and reassures the potential buyer just coming to the platform "we won't forget you six months or a year from now." I still find it impressive that my wife's creaky first-gen circa-2007 iPhone has been offered every single OS update just like the new models. (Not all functionality is supported, of course, as the necessary hardware, e.g. GPS, compass, etc., may not be present.) Can Microsoft, HTC, Samsung, or any other OEM say the same for any Windows phone released in 2007?
IE Mobile 6, for example, was actually released to OEMs to include in late WM6.1 devices, but was never made available as a download for existing 6.1 users. Sure, it was huge, and few devices had the free storage memory to utilize it perhaps, but what would've been the harm in offering it for the few that could have utilized it? The gesture in and of itself would've been a powerful message about support and obsolecence.
I'm certainly not laying all the blame here at Microsoft's feet- the bulk of the problem is with the OEMs, who have little financial incentive to support older models, but Microsoft certainly could help by insuring that their own applications support devices as old as is practical to. I have a hard time believing that the Facebook app, for example, couldn't have been made WM5 with just a little effort. Third-party developers are careful to offer as much backwards compatibility as possible to increase their potential market, why doesn't Microsoft follow that lead themselves to eliminate the doubt and uncertainty that buyers might have when comparing Windows phone to far more easily updated platforms like iPhone and Android?