Send your name to the Moon on LRO (Lunar Reconnaissance Orbiter): the deadline is 27 June 2008
Send your name in search of a habitable planet on the Terrestrial Planet Finder mission, KEPLER; the deadline is 1 November 2008.
On the KEPLER mission you can write some words as well. You can also download a starmap showing its field of search plus the location of those exoplanets already discovered. KEPLER is linked to International Year of Astronomy 2009.
More info: