The new iPad Verizon version also supports AT&T GSM network. However, the GSM version of the new iPad can’t perform on Verizon network.
A users who purchased Verizon iPad has confirmed on a Mac Forum that the new iPad includes a SIM slot to insert AT&T microSIM chip. According to this user the only thing required to activate the AT&T data plan is to adjust the APN parameters under Settings.app.
I was one of the first to obtain a Verizon iPad and can happily confirm that this is allowed! I used my AT&T iPhone 4S sim card and took out the Verizon sim, and data worked! You must apply the AT&T APN carrier settings before this works though.
It seems Apple is issuing unlocked Verizon models finally. However, the same is not possible on AT&T GSM models due to hardware limitations. The Verizon iPad will support only non-LTE bands (EDGE, 3G, HSPA+ 4G) on AT&T network.
Here is video showing how to swap the SIM cards and use the GSM network on iPad 3.