Just let them know that the technology will be here within the next 1-2 years or so, readily useable within 2-3 years, and unless they can find a time machine, then they just have to deal with that reality. The next revision of EVDO will support simultaneous V+D, as will UMTS/HSDPA devices. But until any of these technologies are A) widely spread around the United States (that won't EVER happen), and B) supported devices are available from RIM, then they'll just have to deal with that VERY minor inconvenience.
...you can quote me, if you'd like.
P.S. - Luckily your market will get these technologies. Just think about all of the CEOs and Execs in Nowheresville, USA who won't likely see this technology within the next 5-10 years, if ever.