PostPath has positioned its Linux-based server as something that can either replace or augment an existing Exchange server; it interoperates natively with Exchange and Outlook, and also with Active Directory and other systems like the BlackBerry Enterprise Server (BES). This means it fits pretty seamlessly with an enterprise's broader e-mail infrastructure, both on the directory side and also for enabling mobility, which is of course a huge concern for most enterprises.
Cisco says it plans to integrate the PostPath technology with its software-as-a-service (SaaS) WebEx platform, suggesting that it may be preparing a serious push for network-hosted Unified Communications. That seems like a reasonable play, but I really think the fact that PostPath functions as a part of the Exchange infrastructure means that Cisco can take this onto the enterprise premises if they choose. PostPath gives Cisco something with which to get in front of the people who until now were likely to default to an OCS migration strategy. Once in, Cisco could use PostPath the same way that Microsoft is positioning OCS: Maybe you're not ready to ditch your major platform for this new thing (PBX for OCS; Exchange for PostPath), but you can easily add the new thing for tactical purposes and see where it goes from there.
I don't know if Cisco will go this route or if it will make inroads if it does. But it seems like it now has that option.