To my knowledge, Verizon doesn't have any LTE international global roaming agreements. 3G, yes, they have negotiated rates and billing with carriers in many countries.

AT&T does offer some global roaming agreements with LTE
http://www.att.com/gen/press-room?pi...rticleid=37438

The upside is that the Verizon iPhones come unlocked, so you can purchase a Japanese LTE data sim. Downside - It wouldn't necessarily be that much faster than your included 3G roaming as prepaid visitor data sims in Japan are all from MVNOs which get throttled. Encroaching on the "PrePaid SIM Japan " thread a bit, you'd need to rent a docomo sim to get the "domestic subscriber experience" http://www.jcrcorp.com/mobile/mobile_rental_simcard.htm