ok, so I think I might have figured it out
All iPads are unlocked, BUT the two '4G' iPads have a different model number:
iPad3,2 - CDMA/GSM iPad (aka Verizon iPad)
iPad3,3 - GSM iPad (aka AT&T iPad in the US)
so
my guess is that since the verizon iPad (iPad3,2) is unlocked, it will accept accept any GSM sim card including the at&t, but it won't let you sign up for an at&t plan on the verizon iPad (iPad3,2) because at&t doesn't recognize (or actively blocks) the iPad3,2 model number.
This should explain why a foreign iPads (iPad3,3) should work in the US with an at&t sim card since the rest of world will only be getting the iPad3,3 models.
Kind of makes you wonder if the verizon iPads (iPad3,2) will have any issues activating a local GSM sim in another country even thought it's unlocked