See http://arxiv.org/abs/0810.5128 for the gory (and they are very gory!) details.
Basically, it's unlikely that the inflaton field has the same value in the two bubbles. When the bubbles collide, the bubble with the lower energy inflaton field will see the collision as a hot spot, and the bubble with the higher energy inflaton field will see the collision as a cold spot.
The paper also calculates the geometry of the effects seen in the CMB. I couldn't follow much of this, though the general result is that the collision will have SO(2,1) symmetry and this creates a cone. We see see a cross section of this that is a circle.
This post imported from StackExchange Physics at 2014-03-12 15:23 (UCT), posted by SE-user John Rennie