updated 01:35 pm EDT, Fri March 16, 2012
iPad HSPA works on either US carrier
New experiments from a MacRumors forum member has confirmed that the new iPad's cellualr access isn't carrier-locked. Although frequency differences prevent LTE access, a Verizon iPad will use an AT&T micro-SIM to connect at HSPA+ speeds. The change requires changing APN (network profile) settings, but it doesn't require jailbreaks or forced unlocks.
Apple has usually kept earlier HSPA-based 3G iPads unlocked, but the inherently incompatible nature of EVDO 3G on Verizon made it technically impossible to jump carriers. The new cellular chipset not only needs a SIM card by nature for LTE but is a world roaming chipset that supports HSPA+ as well.
A reverse instance, of a Verizon SIM in an AT&T iPad, won't work. Verizon only provisions SIM cards for an LTE network, and EVDO isn't usually switchable, even if exposed in the hardware.