Groups of capabilities: users with multiple roles?

Question

I'm pretty sure I understand the roles and capabilities setup in WordPress: granular capabilities, grouped together in roles that can be assigned to users. Code should check the granular capabilities, not the roles (because the capabilities for particular roles can change). Roles are not necessarily hierarchical (though the default roles are).

Is there any way of assigning multiple roles to users? Alternatively, having a number of groups of capabilities, and associating one or more group to a user? The way my site works, there are a number of obvious responsibilities: updating web pages, moderating forums, updating the events calendar, and so on. Each responsibility has a group of capabilities that are needed in order to perform the tasks associated with it. I'd like to enable a user to perform one or more responsibilities. So user A could update web pages and the events calendar but not moderate the forums (not nearly tactful enough), but user B could moderate forums, update the events calendar, but isn't allowed near the web pages.

Short of defining a role for each possible combination of responsibilities, is there any way of doing this?