Astrobiology magazine points out a fascinating side-effect of the the Space Shuttle Columbia tragedy. As the space shuttle broke up, its entire cargo, including scientific experiments, fell to Earth. During the recovery search, it was discovered that a colony of C. elegans roundworms which had been on board the shuttle actually survived the fall to earth despite travelling at anywhere from 600-1000 km/hours and a calculated impact velocity of 45 m/s.
The C. elegans had been growing inside sealed cannisters designed to later be incorporated into a long-term automated spaceborne experiment. They were effectively embedded in a soft gel.(In earth-based laboratories they grow on top of bacterial plates, eating the bacteria.) But the cannisters were not at all designed for impact.
Apparently the worms did not get extremely hot. The inner linings, made of polystyrene (melting point about 80 degrees C), were not damaged, and in any case earthbound C.elegans die above about 40 degrees C.
The first potential significance of this is that even multicellular organisms could possibly survive planetary impact. Space probes sent from earth should be well sterilized to prevent forward contamination of outside bodies; but also, allowing spacecraft to break up in the atmosphere of other planets may not be sufficient to kill everything on board.
A more speculative possibility is that natural objects such as meteorites could transfer multi-cellular organims between worlds. For example, as the authors point out,
(free abstract with link to pdf) the Martian metorite ALH84001 (his friends call him AL) was also not heated past about 40 degrees. Of course, once you've survived impact you'd need to survive whatever you landed in. The main danger to the current C.elegans was that their culture plates were being overgrown by (terrestrial?) mold.
UPDATE: Of all people, the hilarious David Sedaris had already written about these worms. In his short story "Can of Worms" in the collection Dress your family... , he overhears about the space worms and wonders about their perspective. You can hear an audio version of the story here at This American Life.