British astronomer David Darling(1) shared his thoughts on the possibility of extraterrestrial life in the solar system. According to him, scientists have tended to focus the search for alien intelligence by scanning radio signals from other stars. Darling, however, believes researchers should be looking for signs in our own solar system because it is "prime real estate for life."
Darling discussed the difficulty in exploring outside of the solar system, pointing out that Earth's nearest extra-solar star, Alpha Centauri, is much too far away for humans to reach. He suggested the future of interstellar travel would most likely involve unmanned, robotic vehicles. Interestingly, Darling thinks some UFOs could be automated probes sent by intelligent beings to study life on Earth.
Still, Darling estimates a 50/50 chance that aliens are already on Earth, perhaps tracking our progress as a species. He also theorized that a more advanced alien race may have left behind coded messages in our "junk DNA" to be discovered after humanity reaches the proper technological level.

1. http://www.daviddarling.info/

Stardust Project

In January 2004, NASA's Stardust spacecraft successfully rendezvoused with comet Wild 2, collecting particles released from its surface and interstellar dust. The Stardust comet sample is scheduled to return to Earth in January 2006.
Scott A. Sandford, a NASA research astrophysicist, said he hoped the Stardust Project would help scientists determine "what fraction of [cometary material and dust] is organic, what kinds of organics they are and what possible role they may have played in life's emergence on earth." Read more at NASA.gov(1).
NASA graphic shows Stardust encounter with comet Wild 2.

1. http://stardust.jpl.nasa.gov