Just saw an article about a green home in popular science where they are testing out a phase change drywall.   National Gypsum is experimenting with ThermalCore, a drywall embedded with wax filled capsules.  The wax liquiefies around 73 degrees which absorbs thermal energy and later releases it as temp drops.

Pretty cool stuff.

There are cost issues with phase change materials but more important is the increase in temperature needed to make it useful - what air temperature would it need to actually get enough heat flow into the wall to be usefully quick - 80+F? And out - 66F? IMO, most interior thermal storage belongs in active systems.

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Hi,
Here is a good page about phase change drywalls.
http://www.doityourself.com/stry/phasechangedrywall