Keep in mind that it seems at this point it's not going to be a world-wide release (e.g. all stores get it at the same time). It looks more like it'll be regionalized. Especially since there are hardware keyboard differences and differences in the regulation of wireless channels (e.g. 1-11 in USA and 1-13 in EU). So the USA might get it before it's released in the EU or vice versa (though it seems by release dates, it's the EU countries that are reporting dates being pushes as far back as December).

Then size of the companies purchasing the product from Nokia (e.g. Amazon versus a smaller retailer) and whoever has contracts for the n900 (none in the USA but some countries have the n900 on contract deals) also plays into account as to which retailers get the lion share of the product.