Monster Hunter 3 Ultimate implements region-locked servers. As a result, you'll only be able to play the game online with others in the same territory as you.
Let's say you live in the US. Unfortunately, the region-lock will prevent you from experiencing Ultimate with gamers in Europe and Japan.
Capcom community specialist Yuri Araujo, a noted Monster Hunter fan, is none too pleased with the news. While Araujo noted that he'll be passing along "the demand and feedback"
I'm not thrilled with the servers being split either... I am definitely aware of the demand and feedback, not only in this thread, but also from threads before finding out these bad news; so be sure that I will pass it along.
There is no guarantee this will change though. =/