>But doesn't fast-track generally imply priority review?<
An unmet medical need generally results in a priority review, whether or not there is a Fast Track designation.
In the case of ATryn, the unmet medical need stems from the lack of consistent availability of Thrombate, the plasma-derived antithrombin product from Talecris. The FDA could also deem the enhanced safety of a recombinant product (ATryn) to be an unmet need, but this may be a stretch.