Not all smartphones have Wi-Fi, and not all allow you to tether them to laptops and connect to the Internet. Certainly getting wireless data that way seems the natural way to do things, rather than using a wireless modem. If you already have a 3G smartphone, why should you have to bother with both? As long as your wireless carrier allows you to tether, this seems to be the best way to do it.
For starters, the software will only work with Symbian S60 and Windows Mobile smartphones. With EVDO-based smartphons, such as those from Sprint or Verizon wireless, incoming calls will disconnect the data call and pass the voice call through to the phone. If you're using AT&T's HSDPA, however, voice calls will be passed through the connection to the phone without breaking off your data session. So you'll be able to surf and chat at the same time.
A demo version of the software is available on a trial basis. TapRoot doesn't plan to sell directly to consumers, though, and will be targeting carriers as its potential customers. You can bet that carriers will want to charge to provide this type of service to smartphone users.