Say you have several webparts, one as a controller and several which take information from the controller and act on it. This is fairly easy to model using the Consumer/Producer interface introduced in ASP 2.0.
How would you be able to add interactions the other way around whilst still maintaining the above?
A simple example would be: the user enters information into webpart A which performs a search and the results would be displayed on webpart B. Webpart C allows you to filter the results which should trigger webpart A to re-submit the query and hence update the results in B.
It doesn't seem possible to do in WSS 3.0 because you are only allowed 1 interface to be used in all of the connections at any one time.
Does this even make sense ? :-)
A quick and dirty solution to enable arbitrary control communication is to use recursive find control and events. Have the controls search the control tree by control type for what they need and then subscribe to publicly exposed events on the publishing control.
I have previous used the trick to enable standard server controls to find each other when embedded in CMS systems from different vendors to avoid a specific communication API entirely.