Being a geek, I only care what it can do for me, not how many are walking the street. The only reason to care about that at all is if more walk the street then Nokia or someone else is more likely to make another device with similar hackability but more powerful hardware sometime down the road.

You don't need to sell tens of millions to reach that goal. A niche market of a 500K or a million can be pretty profitable especially if development costs are relatively low because of the use of open source.

My understanding is that Nokia sold quite a few more than they expected and consider the N900 a success. So hopefully that means there will be some follow on in the future.