There is a Mars-related paper in the June 1, 2007, issue of Science. The embargo isn't lifted for a couple of more hours, so the direct links below are not currently active. However, you can get a brief synopsis here.

Mars: A New Core-Crystallization Regime
Andrew J. Stewart, Max W. Schmidt, Wim van Westrenen, and Christian Liebske
Science 316, 1323-1325 (2007)
High-pressure experiments imply that Mars has an entirely liquid iron-nickel-sulfur core that, unlike Earth's core, will not form an iron-rich solid inner region as it cools.
Abstract
Supporting Online Information

See also:

Mars’ Gooey Core is Freezing
By Dave Mosher, Space.com
posted: 31 May 2007
02:03 pm ET

Lab study indicates Mars has a molten core
16:53 31 May 2007
NewScientist.com news service
David Shiga

That is quite interesting. The core could possibly be molten, yet there is no active magnetic field. But then, these people might not have a solid handle on the sulfur concentration, so the core might not really be molten. I guess it just shows how much we really don't know about our solar system.

It kind of reminds me of the recent studies of Mercury's magnetic field.