Deep in the Zune.inf driver files included in the update are a list of compatible USB hardware IDs - numbers that identify different devices to a computer. In the list are the regular three types of Zune MP3 player, but also three identifiers marked "Phone". That means that Microsoft is planning at least three distinct phone devices that'll interface with its Zune desktop software.
There's already been talk of Windows Mobile 7 having a bunch of Zune-y features, but these IDs mean that there'll be three distinct devices produced in-house by Microsoft. USB regulations state that these IDs can't mask a different device.
Given we're just weeks away from Mobile World Congress, in which Pocket-lint will have a full presence at, we wouldn't be remotely surprised if Microsoft finally unveils the Zune Phone in Barcelona. Stay tuned for more.
Oh, and on a side-note, if you're in the UK, we'd advise against installing the update for now. It appears that Microsoft has disabled some features for international users - like SmartDJ and images being pulled down for artists in the Quickview mode.