That's exactly what we're planning to do!
When I first received an electronic 'Save-the-date' notice, I hesitated before deciding whether it seemed tacky, but then my intended pointed out that the 'save-the-date' card itself seem to be a fairly recent invention... I don't recall ever having seen anything like them at all until this year, when I was inundated with them. Why not combine the ease of e-mail with the happenin' trend?
I love your site! It's been helpful to us, and quite entertaining too!
Francis and I had a less formal wedding than I imagine you're having, but here's what we did: we called up people like relatives, and then sent out save-the-date e-mail to all of our attendees we had e-mail addresses for. If it bothered people, I don't know, because all we got were dozens of delighted e-mails from friends and no complaints.