There have been occasions when I haven't known the answer to questions, but known people who did. When I think of it, I usually try to send them a link to the question.
It could be useful to have a way to refer people directly from the question page.
Know someone who might be able to answer this question?
Enter their email address here: _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
We'll send them a note, and if they answer you'll earn a few extra points.
We won't save their address, and we won't ever send them unsolicited email.
Other services like Aardvark allow you to refer friends, and it seems to work really well -- users seem to use it, their friends often answer questions referred to them, and my impression is that people don't generally think of it as spammy.
I think it would have the benefit of bringing tough questions to the attention of people likely to know the answers, and it would also extend Stack Overflow's realm of influence to people who might not otherwise know about the service. Using referrals would advocate Stack Overflow in the best possible way: by people who already know and like the service, to people they know and likely respect.
I think this is a feature I'd probably use, and I imagine others might too.
PS: There's a small concern about disingenuous users potentially gaming the system by using multiple accounts if you were to offer a point reward for referrals. Hopefully this would be an edge case, and it probably wouldn't be too hard to detect and dissuade by limiting the number of questions that can collect points when sent to the same user's email address.