I bought an iPad Mini on the AT&T network. I then purchased a separate T-Mobile nano SIM because I wanted the 200mb free data.
When the iPad Mini arrived, I simply removed the AT&T SIM and inserted the T-Mobile SIM. When I started up the device, it found the T-Mobile network right away and gave me the free data.
Hence: iPads are UNLOCKED, period. Doesn't matter how you buy it.
The only sticking point is that until the most recent revisions, iPad cellular radios were specific to certain carriers (GSM: AT&T or T-Mobile; CDMA: Verizon; not sure about Sprint's network, but I think it's CDMA)