Save-the-Date notices (STDs for those of us who are brave enough to acronym-ize them) are becoming more and more a standard part of the average wedding stationery suite, as evidenced by this article (oops, the original link that I posted is dead, so here is a new one!) and a slew of others I've seen recently. I think as wedding planning steadily spins further and further out of this universe, as well as the massive increase in destination wedding planning, save-the-dates are truly becoming necessary for many brides and grooms.
The choices for save-the-dates really run the gamut from magnets to stickers to, (of course my personal fave!) postcards! (online store coming SO soon).