The twitterverse (etc.) is alive with EU folks suggesting this should come with iPhones sold there, to really sort of meet the spirit of the rule. You can already buy such adapters, just not in the right color with an apple on it. The point was to not have to buy something new everytime you change devices, or not having to have multiple chargers for multiple devices. Having multiple adapters isn't optimum.
Personally though I would have liked MS to have considered this as opposed to standardizing just on microUSB for WP7. Over previous proprietary connectors, on WM, I could get video, audio (in and out), sometimes serial for console connections, and such. Lost all that with microUSB. iPhones still offer most of that and HDMI as well.
To me the ideal would be to allow a proprietary connector for additional I/O, but require a standardized charge/sync connector, though wired sync is becoming a legacy feature quickly. Apple could put the microUSB on one side or the other of the current dock connector, MS could have specified the Zune connector for WP7. There are at least some Zune peripherals around that could take advantage of it. Guess there isn't anything stopping any manufacturer from doing this, just ROI considerations.
Maybe wired connectivity is expected to really be a thing of the past with AirPlay, DLNA, BT, etc., but there needs to be much greater adoption of those technologies in the target device than there is now to abandon the cables today, IMHO.
Sometimes you are the anteater, sometimes you are the ant.