Checked the area from a flight between Vegas and OC the other day, and then a much closer look on Google Earth looks promising. Its past Baker, on the south side of the 15 Freeway. I plan a trip on 9/29/08.
Though I don't have a map in front of me, the lava beds and cinder cones are concentrated about ten miles south of Baker.
FUN JUNKIE is correct, lava is VERY HARD on rubber so try and maneuver around the mounds of lava as much as possible. You can easily chew up a new set of tires in a day of running on lava.
The Mojave Road runs through part of that terrain you're interested in and I think that's what FJDAVE refers to. There are some hidden coves in that area that make for great camping and exploring brings rewards. Much of that area has been previously impacted so you don't have the concerns about crypto soil or other undisturbed ground that you might have in another area. There are also mining roads that criss-cross the area and it's a fun day trip from SoCal or Las Vegas.
In addition to the lava, there is DEEP sand in that area depending on the time of year. It's not uncommon to find dunes across roads-less-traveled where you sink to the skids, so if you're not driving a 4X4, bring a friend in a second rig with a rescue strap (You can bring AIR2AIR because he has a wazoo rescue strap now).