It's hard to say. Obviously you can't compare Xbox 360 or PS3 sales, even though they compete with a lot of the ATV functions. The Boxee Box and Google TV devices are just releasing now. The only real comparables are Roku, WDTV, Squeezebox, and a host of smaller devices.
It's still such a poorly defined segment. So many devices capture some of its functionality, such as gaming consoles and some blu-ray players and even TVs. I think that particularly with the release of the Google TV, competition in this segment will heat up and we may see a market explosion. This time next year I suspect sales numbers will be a lot more interesting.
Still 250,000 is a solid head start for them.