Sprint will sell its Overdrive, a router that handles 3G and WiMax, starting 10 January: The device, $100 with a two-year contract, works with either 3G CDMA or 4G WiMax, preferring the 4G network. It's $60 per month, which includes unlimited WiMax usage and 5 GB of 3G. The device appears to be nomadic, rather than mobile: that is, intended to be taken from place to place, but not used while in motion. (It's unclear whether or not there's a car adapter included.)
Any 802.16e device should be able to handle full mobility. The press release even indicates "use Overdrive to keep passengers entertained in the car." I'm actually surprised that WAN-to-WLAN devices like the Overdrive and the MiFi aren't more popular than 3G/4G USB dongles or embedded solutions. The flexibility that they offer is remarkable.