Bluetooth dongles will definitely work. One thing they did very right was specify the hardware interface as well, so all Bluetooth dongles use the same hardware interface, and thus the same driver. I'm not sure that the team will be building the Bluetooth stuff into the kernel they ship, but worst case you can build your own.
The dongles can be had for a couple bucks from DealExtreme or eBay, and for the price, I have one in all of my computers. It's just convenient.
I have tried a Bluetooth dongle - worked fine. I think I needed to rebuild the kernel but I expect it to be built in on release.
Principal Software Engineer at Raspberry Pi (Trading) Ltd.
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