It turns out that this is a roam area, possibly using Verizon. The gray blob between Columbus and Jackson in the coverage map at right is classified as “mobile broadband roaming” on Sprint’s Coverage Tool. (Verizon’s own coverage map doesn’t have a special broadband area that corresponds to the gray blob, so it may not be Verizon after all?)
Once I got into Flowood, MS, I reentered Sprint’s pokey 1X network. Argh.
On a whim, I forced my phone into roam mode, and I get EVDO even in Sprint’s 1X area! So it appears that if you only have 1X access, you may still be able to get high speed EVDO by forcing roaming.
UPDATE: I got EVDO on I-20 all the way to Van, TX by forcing the phone to roam mode. Otherwise, it would have been 1X the entire way.