Fat tires can have a bit of wobble to them. Most important is to check that the beads are properly seated uniformly around the rim. There's usually a line on the tire near the rim that should be evenly spaced from the edge of the rim all the way around. Sometimes with new tires you need to inflate them to a bit higher pressure than your normal running pressure (but not over max rated) to get the beads to seat and then bleed the pressure down to your usual.
Of course getting a tube caught/pinched between a bead and rim can cause the tire to wobble too, and can lead to a blow out.
I'd also inspect the tire carefully looking for torn cords, significant damage, or seriously deteriorating rubber.
Edit. Holy cow. In the time it took me to respond to the OP 4 additional posts popped up.
Last edited by Looigi; 10-29-12 at 08:18 AM.