No, olivine might mean that SMALL portion of Mars hasn't been wet in a long time, but only if you assume volcanism hasn't been active on Mars for a long time. A key point though is that this is completely at odds with the most up to date data on volcanism on Mars.
William Hartmann is the leading expert on volcanism on Mars and he believes volcanism is still currently active on the Red Planet:

Study Points to Recent Volcanic Activity.
posted: 07:00 am ET
21 July 2000
"The young flows show Mars "has much more recent volcanism and subsurface heating than anyone expected," Hartmann said. "If there has been volcanism throughout [4.6 billion years of] Martian history up until a few million years ago, it’s unlikely it stopped just before we got there. So it’s probable volcanic eruptions will continue in the future."
http://www.space.com/scienceastronomy/mars_lava_lead_000720.html

And here's another article that makes the same point:

Mars: recent geological activity
Mars: a geologically active planet
Karl L Mitchell and Lionel Wilson
Karl L Mitchell and Lionel Wilson summarize the case for martian geological activity in recent times.
Abstract
"Recent and continuing missions to Mars are showing that the Red Planet may be more geologically active than previously thought. Volcanoes and erosion by running water have shaped the surface. And evidence is growing that fluvial and possibly volcanic processes have been active in the very recent past."
Astronomy & Geophysics
Volume 44 Issue 4 Page 4.16 - August 2003
doi:10.1046/j.1468-4004.2003.44416.x
http://www.blackwell-synergy.com/links/doi/10.1046/j.1468-4004.2003.44416.x/abs/

Also, key here is that even if this recent volcanism hasn't occurred in that SMALL portion of Mars where the olivine was seen, since the GRS readings show some amount of near surface ice over virtually the entire planet, including the equator, this recent volcanism would have caused sufficient heating and atmospheric pressure to allow this ice to melt to liquid form.

It is notable as well that some areas where the volcanism is regarded by Hartmann et.al. to be recent such as Tharsis and Eysium have actually been found to be anomalously HIGH in hydrogen by GRS, perhaps to the extent that they may contain debris covered glaciers.

Bob Clark