Preview at WWDC likely in June, followed by September launch.
New Verizon iPad Runs on AT&T 3G Networks with AT&T Micro-SIM
As noted by jsnuff1 in our forums, the new Verizon iPad does indeed work on AT&T's network when equipped with an AT&T micro-SIM and the device's carrier settings are updated to support connecting to AT&T.
Big question on everyone's mind buying the Verizon iPad was whether data from AT&Ts 3G network would work on this iPad, or if Verizon/Apple would block this.It is important to note, however, that a Verizon iPad will be limited to non-LTE bands (EDGE, 3G, HSPA+ 4G) on AT&T. The Verizon iPad hardware supports only the 700 MHz band for LTE to match the carrier's technology, while the GSM version sold for use on AT&T's network supports both the 700 MHz and 2100 MHz bands as used by that carrier.
I was one of the first to obtain a Verizon iPad and can happily confirm that this is allowed! I used my ATT 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.
Initial pre-orders of the new iPad are arriving today, while retail outlets in ten countries are offering in-store purchase for those still looking to purchase an iPad on launch day.