Verizon iPad roaming capability

Anandtech believes that the SIM slot is unlocked on the iPads so in theory yes we should be able to throw in an AT&T SIM and use 3G data. We will have to wait for someone to try it to confirm though.

This is from Apple's Tech Specs for the new iPad:

Wi-Fi + 4G for AT&T model: LTE (700, 2100 MHz)3; UMTS/HSPA/HSPA+/DC-HSDPA (850, 900, 1900, 2100 MHz); GSM/EDGE (850, 900, 1800, 1900 MHz)

Wi-Fi + 4G for Verizon model: LTE (700 MHz)3; CDMA EV-DO Rev. A (800, 1900 MHz); UMTS/HSPA/HSPA+/DC-HSDPA (850, 900, 1900, 2100 MHz); GSM/EDGE (850, 900, 1800, 1900 MHz)

So, aside from the absence of 2100MHz LTE in the Verizon model and the lack of CDMA in the AT&T model the HSPA+ and GSM bands appear to be identical between the units, so, yes, in theory it should work.

The Verizon iPad is compatible with AT&T’s HSPA+ network, but the AT&T’s iPad model won’t be able to hop on Verizon’s network.

So if the iPad ships SIM unlocked, as it had before, users can swap in a micro SIM from AT&T and use Verizon’s 4G iPad on AT&T’s 3G and 4G HSPA+ network–they just can’t use a Verizon 4G iPad on AT&T’s LTE network.

For international roamers, LTE roll-out in Europe is expected to use the 800MHz, 1800MHz and 2600MHz bands, so neither US iPad will be able to get LTE there.

Hong Kong: 2.6GHz band, Japan: 800MHz and 2GHz bands, Singapore: 1.8GHz and 2.6GHz. So no LTE there either.

Here's a useful site for other countries and 3G/4G
http://www.worldtimezone.com/gsm.html