I was an employee at an aerospace company and happened to travel to Kennedy Space Center on the day that Ares I-X launched. Knowing that there was a possibility that Ares I-X might launch while I was there, I prepared by creating this stamped envelope with Ares I-X artwork. Fortunately for me the flight happened while I was there, and after the launch I immediately took the envelope over to the KSC post office and had it cancelled.
Photos: the Ares I-X postal cover, a picture of the cover getting cancelled at the KSC post office, a close-up of the cancel, and a picture of the launch that I took.
Envelope is 3 5/8" x 6" and ships in the protective archival case that it's been in since that day.