Can you purchase the iPad "unlocked" or is it tied to a wireless carrier?
iPad models equipped with wireless mobile data connectivity (3G or 4G+LTE), regardless of generation, are "unlocked" and not tied to a carrier in the United States.
However, just because the iPad is sold unlocked does not mean that any specific version of the device will work on all carriers. Paying attention to supported frequency details is important to avoid disappointment, and perhaps even more so on the used market.
Original iPad & iPad 2
It is worth noting that even though the GSM-based original iPad and GSM-based iPad 2 models are not locked, the devices do not support the Verizon CDMA 3G network nor does either support the 1700 MHz UMTS/HSDPA frequency used by T-Mobile for 3G in the US. Likewise, the CDMA-based iPad 2 does not support GSM networks like AT&T's and is not intended for use on networks other than Verizon's in the US.
Even though the original iPad and GSM-based iPad 2 models are not locked to AT&T Wireless, the devices are not hardware compatible for 3G with other US wireless carriers. However, either should operate via the slower 2G standard over T-Mobile (assuming that one is willing to prepare a "trimmed" Micro SIM card).
CDMA authentication is more complicated than just switching SIM cards on a GSM network, and as a result, using a CDMA-based iPad 2 on a network other than Verizon's in the US -- like Sprint or Cricket -- was a challenging task. However, following the hack of the CDMA-based iPhone 4 to run on Cricket, it's not surprising that clever individuals hacked the CDMA-based iPad 2 to run on Cricket as well.
iPad 3rd Gen
Like its predecessors, the iPad 3rd Gen line is not locked to a carrier.
However, the 4G+LTE version for AT&T specifically supports the 700 MHz and 2100 MHz 4G LTE frequencies as well as UMTS/HSPA/HSPA+/DC-HSDPA (850, 900, 1900, 2100 MHz) and GSM/EDGE (850, 900, 1800, 1900 MHz). This model was sold around the world for a variety of carriers, but it only supports 4G LTE frequencies in the US (on AT&T) and Canada (on Bell, Rogers and Telus). By contrast, it only will work with 3G networks in other locations, which led to legal troubles for Apple in some countries.
In the US, the Verizon version specifically supports just the 700 MHz 4G LTE frequency. It also supports CDMA EV-DO Rev. A (800, 1900 MHz), UMTS/HSPA/HSPA+/DC-HSDPA (850, 900, 1900, 2100 MHz), and GSM/EDGE (850, 900, 1800, 1900 MHz).
The iPad 3rd Gen version for AT&T also can use T-Mobile, but only on the company's slower 2G network.
iPad 4th Gen & iPad mini 1st Gen
Like earlier iPad models, the iPad 4th Gen and iPad mini models still are not locked to a carrier. However, LTE capabilities are different and each configuration is limited to carriers that support a particular LTE band.
Specifically, the A1459 iPad 4th Gen (Wi-Fi/AT&T/GPS) and A1454 iPad mini (Wi-Fi/AT&T/GPS) mostly intended for use in the US on AT&T and in Canada on Bell, Rogers and Telus -- support LTE bands 4 and 17 as well as GSM/EDGE (850, 900, 1800, 1900 MHz) and UMTS/HSPA+/DC-HSDPA (850, 900, 1900, 2100 MHz).
In the US, the A1460 iPad 4th Gen (Wi-Fi/Verizon & Sprint/GPS) and A1455 iPad mini (Wi-Fi/Verizon & Sprint/GPS) are intended to work only on Verizon and Sprint. However, these models also were sold by a large number of other carriers around the world. Specifically, both support 4G LTE bands 1, 3, 5, 13, and 25, as well as CDMA EV-DO Rev. A and Rev. B (800, 1900, 2100 MHz), GSM/EDGE (850, 900, 1800, 1900 MHz) and UMTS/HSPA+/DC-HSDPA (850, 900, 1900, 2100 MHz).
In general, the A1460 iPad 4th Gen (Wi-Fi/Verizon & Sprint/GPS) and A1455 iPad mini (Wi-Fi/Verizon & Sprint/GPS) are compatible with more carriers worldwide.
iPad Air & iPad mini Retina (2nd Gen)
Like all previous iPad models, the current iPad Air and iPad mini 2nd Gen still are shipped unlocked. Even better, there is only one cellular capable model of each device and it provides much more global coverage than earlier iPad tablets.
Specifically, the A1475 iPad Air (Wi-Fi/Cellular) and A1490 iPad mini (Retina/2nd Gen - Wi-Fi/Cellular) support a whopping fourteen LTE bands -- 1, 2, 3, 4, 5, 7, 8, 13, 17, 18, 19, 20, 25, and 26 -- in addition to CDMA EV-DO Rev. A and Rev. B (800, 1900 MHz), GSM/EDGE (850, 900, 1800, 1900 MHz) and UMTS/HSPA+/DC-HSDPA (850, 900, 1700/2100, 1900, 2100 MHz).
However, curiously, as first noted by PCMag:
Sprint will only activate unlocked iPads purchased directly from Apple [or an Apple Authorized Reseller], where the owner requested a "universal part" rather than a specific carrier unit. . . Unlike the three other large carriers, Sprint isn't required to activate all compatible LTE devices, and the carrier won't accept devices with the other carriers' part numbers.
EveryiPad.com makes it simple to lookup a carrier-specific iPad Air or iPad mini 2nd Gen "Model" in the iOS. For example, the 16 GB iPad Air configuration in "Silver" on AT&T is MF074LL. Presumably, Sprint would decline to activate this model on their network, but you still would have the option of using the device on AT&T, T-Mobile, or Verizon in the US and numerous other carriers worldwide.
*Please Note: The above information was taken by everymac.com*