SpaceDaily has an article on the latest theories concerning the formation and composition of the Martian polar caps.

It's fortunate that much of Mars' available water is locked up in the northern hemisphere. The topography in that region is generally more gentle than in in the southern hemisphere (hence our tendency to land probes and rovers there), and will for that reason be the likely landing site for manned expeditions and early settlements. Since a local supply of water is of critical importance to settlement of the planet, it's a good thing that it will be available fairly close to these potential landing zones.