Just to reiterate a point that a few others have made: the presence of water ice at the surface of Mars has been understood since at least the 1970's for high latitudes. This goes for parts of the polar caps (also made up of CO2 ice), and the seasonal frosts that are known to coat the very study area visited by the Phoenix lander.
Here's a snippet from an abstract of an article from 1982 (Journal of Geophysical Research, 87:367-370): "A new reflectance spectrum of the Martian north polar cap is analyzed, and it shows water ice absorption features. This evidence confirms the result of the Viking IRTM and MAWD experiments, which indicate that the north residual polar cap of Mars is composed of water ice during the season observed." The Viking 2 lander directly saw seasonal frost in the late 70's, as the Phoenix lander will in the coming months: http://www.jpl.nasa.gov/jplhistory/captions/vikinglander-t.php
The Phoenix results are new in that ice has been directly confirmed for shallow regolith ("soil") materials at the Phoenix site (as opposed to spectroscopically identified from orbit or from the Earth). This is a nice and important result, but is not a huge surprise (the site is known to be seasonally coated with water-ice frosts, and its sediments are distributed in a polygonal pattern that is analogous to what we see at high latitudes on Earth where freeze-thaw action dominates).
Phoenix is a great mission, but let's also give due credit to earlier workers.