I realize that user store its states as session, but i haven't thought about the problem with the same application opened in several tabs (but i guess it is a standard problem when working with session ). I can't see no easy way, if any, to solve this problem.
Overriding CController was not a problem in my special case, but i guess makes it difficult to create an extension of it. Or at least it will be difficult to use it together with other extensions that also overrides CController.
These 2 drawbacks, especially the first one, are much worse than having the url cluttered so i think i will go with Mike's solution.
Will you release it as an extension?