With all the excitement of what Spirit and Opportunity may have found , I wonder what these two probes, MPL and Beagle, would have found if they hadn't failed.
MPL had a robotic arm to dig trenches upto 1 metre deep I believe, far more than the few cm the rovers can dig with their wheels. Maybe it would have struck ice or even water. Beagle also had a device for extracting material from beneath the surface, and the most sophisticated intruments to analyse it.
Fortunately NASA are launching the Phoenix lander in 2007 an updated MPL, and ESA has a pretty ambitious rover in the works too.