I disagree with this - I think that we're more like a single-threaded system. We have the capability to context-switch, but different people are better at it than others. Still, you can only focus on one thing at a time unless you're doing a task that can be given over to a semi-automatic system (driving, for example, might be considered semi-automatic sometimes).
I think that ADD could be akin to having a computer that spent all of its time context switching and never got any real work done.
I haven't see Google Wave or any demo of it, but if it forces people to context-switch too often, it will become distracting. Same as getting 10 IMs and hearing the tea kettle go off when you're trying to focus on writing a Very Important Slashdot Reply. And - oh yeah, when you're trying to work, too.