This seems like a pretty obvious option to have. My old router was able to update my dynamic dns service through dyndns.com automatically. My new router, provided by my ISP, does not. And I do not want to keep my computer on all of the time just to make use of their software-based updater. Since the slingbox would be on all of the time, it seems obvious to me that it should be able to update these types of services directly.
Another way would be to have the slingbox communicate directly with SlingMedia's servers with current IP information which would then be available to the client when the login to their account via the player. Basically, the same way that GoToMyPC works.