I wouldn't count on ever seeing any liquid. Here in New England there have been plenty of mornings lately where have watched thin layer of frost vanish off my car, sublimating directly to vapor, never passing through a liquid stage and never really wetting the surface underneath, despite the air temperature being a bit above freezing.

All you need is for the the relative humidity of the air to be fairly low and you get sublimation above 32F. That's what we see here on days that are clear and dry, when the air temp falls down to the (below freezing) dewpoint and you get a thin frost condensing on surfaces, but then the air warms quickly and dramatically after sunrise. That's exactly what you'd expect on Mars.

Plus, the amount of frost there is *tiny*. In the Viking 2 pictures with frost, I think they were saying the frost layer was only a few microns thick, and I'd assume that's the case with Opportunity, as well. The pic of Opp with frost was taken just 10 minutes after sunrise. That frost was probably gone before you'd have finished your first cup of coffee, let alone still hanging around melting at noon.