Not grounding your equipment properly is just a bad idea - no offense James but using the ground wire in a 3 wire cable as a neutral line, and not grounding anything to the panel is not only illegal, it can be dangerous (not saying I've never done that though!). Going 4 wire gives you the ability to have a proper neutral wire giving both 240 and 120, while at the same time allowing you to ground your equipment to the panel (where its supposed to). I have a few fireman friends who have told me about electrical fires caused by improper or absent grounding. I hadn't realized how bad it can be, especially around liquids. I'm guessing that's why its been in the electrical code for such a long time.
In a new house with new everything to go with it, getting 3 wire is just creating a problem that needs to be solved. Just solve it now and get 4 wire. While you're at it, get nice heavy stuff as well - 8 gauge will give you lots of room to have nice big ass circuits. You'll never have to think about it again...
