HD Radio right now is broadcast over existing radio channels, both AM and FM. It is not truly on-channel, as the IBOC (in band on channel) acronym would suggest, but on the two adjacent channels. This both causes interference to and receives interference from neighboring stations, especially in crowded metropolitan areas. The full-digital is designed to have the signal only on the assigned channel, but it will be roughly a decade before that could happen. Receiver penetration is simply too low, which would cut off the vast majority of listeners from stations if done too soon. That would be suicide.
So, where will we be in a decade? Given the extremely rapid progress of things like streaming audio and iPod-type devices, it's safe to assume that personal multi-function devices will be the norm. These will replace cell phones, iPods and PDAs, plus other functions like GPS and navigational accessories. WiFi will be universal in most metropolitan and suburban areas, corresponding to cell coverage. You'd have to get pretty far into the boondocks to be out of range. Potential satellite augmentation may even fill those gaps. Global coverage will be expected.