The specifications for the new 3rd-gen iPad which was released today are available on Apple's web site, with detailed specifications.
Based on this, LTE support is only offcially supported on AT&T and Verizon in the US, and Bell, Rogers and Telus in Canada. Even http://www.apple.com/fr/ipad/specs/ indicates this:
"La connectivité 4G LTE est uniquement prise en charge sur les réseaux AT&T et Verizon aux États-Unis et sur les réseaux Bell, Rogers et Telus au Canada. Consultez votre opérateur pour en savoir plus."
If you look at the specs on the various country-specific iPad pages, you'll see that Verizon model only appears to be US-only, while the so-called "AT&T version" is what is being offered internationally. That version supports LTE on both 700 and 2100 MHz.
European and Asia/Pacific carriers are using or have announced plans to use other frequency bands, such as 1500, 1800, 2600, and 3500 MHz. Given that, I'd say it's safe to say that LTE for iPad is a US and Canada feature at the moment, at least with the currently available models. 3G data speeds, including enhanced 3G (HSPA+, HSPA-DC, etc.) are available in some areas in Europe, and may provide comparible speeds to what US users will get on LTE.
Summary: It appears that if you buy the current model, it will work on most carriers GSM/UMTS networks around the world, but LTE is a US/Canadian feature at the moment.