What do you mean by federated  portals? From a pure search point of view
you can build a hierarchy of portals  such that result sets are
aggregated the higher up the tree you go...you use Shared Services and
associated search scopes to achieve this...

There is question with regard to interoperability between portals.

* Can a websphere portal host a sharepoint  web part and

* Can a sharepoint portal host a websphere portlet.

Unless something new have come along in the last few months, the answer is "no"
for both. There is a project on GotDotNet to allow SharePoint to consume some
basic portlets, though your best bet is to use web services to consume the
underlying *content* for each and display it using each portal's local rendering
technology.