Cities can be filtered much the same way as Companies (”/jobs-at” and “jobs-in”), since JobberBase 1.7. The overview of Companies in a tagcloud-like way, that has been around since early versions, hasn’t been added to the Cities yet though.

I've posted a tutorial on how to add this: Cities Tag-Cloud Modification.
An example of the running script can be found at: TelefonischWerk.nl.

Implementing it is fairly straight-forward, for support either reply here or at the comments in the blog-post.

Modification Features:

    * Introduces a function that returns all cities that have at least one job
    * Adds an overviewpage (similar to /companies), where cities with more jobs get bigger
    * Can be used for an overview of cities with jobs available on the frontpage