Yes, but it is a manual process. It would be easier to pick up another phone that supports DUN and put your SIM card in it while you are in the car...
The basics are:
Setup on iPhone:
2) Install the terminal application
3) install a SOCKS proxy
Each time you want to use DUN:
1) Configure the SOCKS proxy on the iPhone to route WiFi traffic through EDGE.
2) Create an adhoc wireless network on the Mac.
3) Connect the iPhone to the adhoc network.
4) Get the IP of the iPhone and configure your Mac to use it as a proxy.
5) Surf the net.
Search around and there are detailed directions on setting it all up.
P.S. Tethering is technically a breach of your AT&T contract.