To answer my own question, according to Wikipedia, Mars' surface temperature is between −87 °C and −5 °C, and the atmospheric pressure is around 0.007 atm to 0.009 atm.

And according to this (http://bhs.smuhsd.org/science-dept/marcan/apchemistry/h2opha...) phase diagram for water, sublimation occurs at pressure less than 0.006 atm and temperature less than 0.01 °C.

So while the pressure on Mars might be a little too high, it's pretty darn close. Wikipedia or that phase diagram could be wrong.

Also, I just came across this:

The atmospheric pressure on Mars is just a bit below the triple-point pressure. That's why the textbooks all say liquid water is unstable on the surface of Mars. Any liquid water will rapidly evaporate and the rest, cooled by evaporation, will freeze. Low spots on Mars may have pressures above the triple-point pressure, but are usually far too cold. Even if you did somehow get above 0 C on Mars and into the liquid field, you're so close to the vapor curve that water would rapidly evaporate. Even solid ice would quickly sublimate on Mars except in the very coldest areas.

http://www.uwgb.edu/dutchs/petrolgy/Ice%20Structure.HTM