Don't quote me on this b/c I don't use a MAC. However, I think I remember reading a post a while back somewhere that the regular tethering hack worked. You just have to use bluetooth to connect and not usb.
CAB File here:
http://forum.ppcgeeks.com/showpost.p...64&postcount=1
If you get it working please post back and let us know.
EDIT:
When they told you that you had to switch plans....I know you said your on simply everything, but is a family plan by chance??