But the whole point of this thread is that - usually - Nokia doesn't publish such data for single models.

Of course you're right... and knowing that the N900 outsells the N97 in some online store doesn't mean anything at all.
People don't buy phones online, they get them from their carriers. Even for those who do buy elsewhere, amazon or the Nokia store isn't a valid indicator: As we hear in this forum, it's still pretty difficult in many countries to get the N900 at a local retailer. This may well lead to unusually high sales numbers whereever the device already is available.

So none of the statistics and figures we found answers the question "How many N900s are sold compared to N97s, HTC HD2s or Motorola Milestones or 5800s?"
But seeing quite a lot of indications that the N900 is, at least, not doing too bad (especially for a device that's just entered the market and isn't even available in large numbers everywhere) is comforting. It could have been that we don't find it among any "best selling devices" lists anywhere and that it doesn't make any impact on flickr.