The mini-TES on the rovers has confirmed what was seen from orbit with TES that carbonate exists in small amounts in dust widespread on Mars:

January 09, 2004
Spirit Lowers Front Wheels, Looks Around in Infrared.
"We came looking for carbonates. We have them. We're going to chase
them," said Dr. Phil Christensen of Arizona State University, Tempe,
leader of the Mini-TES team. Previous infrared readings from Mars orbit
have revealed a low concentration of carbonates distributed globally.
Christensen has interpreted that as the result of dust interaction with
atmospheric water. First indications are that the carbonate
concentration near Spirit may be higher than the Mars global average."
http://marsrovers.jpl.nasa.gov/newsroom/pressreleases/20040109a.html

These new readings show these amounts are seasonal. This coincides with the HEND Mars Odyssey measurements showing low amounts of water deposition occurring seasonally, and even at near equatorial latitudes.
Note also the MI imager on the rover shows "cementation" of the dust into crusts:

OVERVIEW OF ATHENA MICROSCOPIC IMAGER RESULTS.
K. Herkenhoff1 (****@usgs.gov), S. Squyres2, R. Arvidson3, J. Bell III2, N. Cabrol4, M. Chapman1, B. Ehlmann3, B. Franklin5, L. Gaddis1, P. Geissler1, R. Greeley6, J. Grotzinger5, J. Johnson1, B. Jolliff3, L. Keszthelyi1, A. Knoll7, P. Lanagan8, E. Lee1, J. Maki5, S. McLennan9, D. Ming10, K. Mullins1, J. Rice6, L. Richter11, M. Sims4, L. Soderblom1, N. Spanovich5, R.
Springer5, R. Sucharski1, R. Sullivan2, C. Weitz12, and the Athena Science Team, 1U. S. Geological Survey, Flagstaff, AZ 86001; 2Cornell Univ.; 3Washington Univ.; 4NASA Ames Research Center; 5Caltech/JPL; 6ASU; 7Harvard Univ.; 8Univ. Arizona; 9SUNY, Stony Brook; 10NASA JSC; 11DLR Institut fur Raumsimulation; 12PSI.
Lunar and Planetary Science XXXVII (2006) 1816.pdf
"Some MI observations of soils
disturbed by the MB contact plate show apparent
fractures, suggesting that cementation of surface
particles has formed a crust. The thickness of this
crust is estimated to be at least 1 mm (the penetration
depth of the MB contact plate) based upon images
taken after the surface was disrupted. Plausibly, and
consistent with APXS spectra of soil-like deposits at
the Meridiani site, the cementation is caused by
precipitation of various salts (e.g., Cl- and SO4-
bearing) that bridge soil particles. Salts in the dust
(unresolved by the MI) may dissolve and migrate into
voids between soil particles in thin films of water
adsorbed onto soil particles. Thin liquid films may
occur in soils when the spin axis obliquity and
atmospheric relative humidity are high enough to
cause precipitation or condensation of water. During
warming events, salts may precipitate on soil
particles as thin liquid films evaporate, weakly
cementing the upper soil surface."
http://www.lpi.usra.edu/meetings/lpsc2006/pdf/1816.pdf

This was speaking of the MI images at Opportunity:

Opportunity :: Microscopic Imager :: Sol 050.

However, such cementation crusts have now been seen by Spirit at the "Home Plate" formation:

Spirit :: Microscopic Imager :: Sol 747.

The report spoke of the liquid water films occurring during high obliquity periods. But the Mars Odyssey HEND data shows water/ice deposition is occurring now seasonally at these sites. Since now both theoretical calculations and actual experiment show liquid water can exist on Mars for short periods, the evidence is now overwhelming that current liquid water, perhaps only as thin briny films, is the origin both of currently forming carbonate dust and of currently forming cementation crusts and these are very likely widespread on Mars.

Bob Clark