Since Pentax does not license the P-TTL digital protocol to third parties, companies like Metz, Sigma, and Promaster must reverse-engineer the way the camera body communicates. Small changes in the protocol when new camera models come out often mean that third-party flashes don't work with the new stuff.
Usually, the problem is quickly resolved, but existing flashes need an update. For Sigma and Promaster flashes, once the update is available, you send your flash back to the factory service center, and they do it for you for free. This is also true of the entry-level Metz flashes, but the nicer models include a USB port. When an update is available, you simply use the MS Windows or Mac OS X software Metz provides to update the flash firmware yourself. This is quick and easy and saves shipping costs and time.
Metz has even updated flashes with small new features in this way.
Note that this isn't really an issue for Pentax-made flashes; so far, whatever changes have been made have stayed within what the existing flashes understand, and presumably Pentax will keep it so. On the other hand, it's a much bigger issue for the "generic-make" flashes, as it's often impossible to get support.
The new Tumax DPT5-series flash is USB-upgradeable, and apparently that feature is also being added to the DPT3-series, although I have not seen a version of the DPT3 with this feature sold in the US yet. (The Cactus AF45 doesn't have it, for example.) Since this feature is new, no updates have actually been made available in this way yet, and it's unclear if one would need to get a Cactus or Vivitar version of the update or if they'll be available from the Tumax web site for all versions.