Engadget Review quote: "If you're not sure of a specific operator's network settings, don't fret: Apple tells us that the Air will detect which carrier you're using and will download the proper settings for you."
Sounds promising.
http://www.engadget.com/2013/10/29/ipad-air-review/?ncid=rss_truncated
Indeed. Anand has similar language in his review. Still, they might be referring only to APNs and not to the built in activation app.
Interestingly, Anand gives the example of purchasing a Sprint iPad and then inserting a T-Mobile, AT&T, or Verizon SIM.
Sprint doesn't seem to offer SIM cards directly---they're still in a very CDMA mindset, in contrast to Verizon, whose 4G LTE devices are interchangeable the way GSM phones are, notwithstanding Verizon's usage of CDMA for voice and 3G data.
One possibility is that the Sprint iPad will effectively be the only truly universal one in the US, since Sprint might only activate Sprint iPads, while other carriers will activate any iPad Air. If so, and if the built in activation app does update automatically based on the current SIM, then the Sprint iPad is undoubtedly the one to buy, regardless of what carrier you plan to use now.