As you said there is WSRP which allow runtime decoupling, so living in the same runtime is recommended for simplicity and performances but it is not mandatory!

Library versioning is solved in Java EE environments where different portlet applications can have their own version of the library.

Doing a portal in a browser using widgets will work fine if you stick to one component model, i.e all your widgets are Google widgets or whatever else. For instance iGoogle works fine because they defined their component model and they only work with it.

Once you start to mix technologies, you open the door to potential issues as the same DOM document is shared between the different widgets (toolkit versionning, prototype modifications done by toolkits, etc…).