sceen311 said it truly. The term "Post-PC" is not only ambiguous, it's misleading. I prefer ubiquitous computing (although it's harder to say). What users (home and business) are increasingly looking for is not one device that suits every need (I doubt there could be such a thing.), but multiple devices that suit various needs but which all have _seamless_ access to the data they need to work together.
Out of the office, smartphones serve as the point of data contact, for getting data in and out quickly and for location-based or location-specific information as well. In the office or at home, tablets can perform simple to mid-sized tasks. The desktop system is still necessary (and will be for the foreseeable future) for the heavy lifting and heavy editing. It's a simple matter of power, screen real-estate, and available input methods. The whole thing only works well when you don't have to sync, transfer, or otherwise move data around to get it onto the device you want to work with.
Right now, I use an Android phone, Android tablet, and desktop PC with gmail, Google Docs, and Google calendar to accomplish this. The added benefit is that I can also share specific docs and calendars with other folks as well.