I sent them yesterday! And I've already had some in-town guests tell me they got them! That's not the USPS I know... :P Unfortunately, they're not 100% off my to-do list because FI has still not been able to get in touch with an aunt and uncle to get their address... still trying, and hoping they won't be offended by a late invite...
Also, the in-laws have just told me the date they need a head count for the rehearsal dinner, and it is two weeks before the RSVP deadline on the response cards. Have your guests typically been responding quickly, or waiting until the last minute?