Inter portlet communication, is not that well defined in JSR 168 specifications. The only way to achieve it is to put the shared data in application scope session of portlets. No specifications were given for a Portlet to communicate to another portlet which is not placed on the same page. Different vendors have implemented the IPC in different ways. Here are some links to the articles which can be very interesting to be read esp. for IPC.
Comparing Various Vendors Implemetation of IPC
IPC Sun Portal
i will be adding more links and sites to his postings as an when more things happens. I appreciate comments to suggest more links.