Supposedly LTE chipsets support falling back to HSPA.
But LTE may be an opportunity for carriers to lock in customers, as apparently a much wider number of bands are supported by the standard, perhaps too may for there to be true universal phones.
VZ and AT&T may purposely demand phone makers to support only the spectrum used by their networks, though they may not be able to force Apple to do so.
EU, one would expect, would be requiring interoperable LTE devices, just as it did with GSM/UMTS.