Wifi tethering works fine on my Mac. Which version are you using? Here's a link to the most recent version:
Downloads - android-wifi-tether - Project Hosting on Google Code
I was able to get tethering to work from my DROID on my iMac and iPad. I have been trying to use Barnacle Wifi Tether but have had no luck. Seems there are many others having this issue as well. I'm currently rooted and running CyanogenMod Blue, Froyo. Apparently there are some issues (not compatible yet? correct kernel?) with Barnacle and 2.2. It was then I gave Wireless Tether a shot. After downloading and installing it, I changed a few basic setting under menu > setup. I enabled wifi-encryption and left the set-up method as Auto. I left the SSID name as the default (you can change this I'm sure, I just left it as the default for a trial run) and changed my password. I then started wifi tether and gave it a few seconds for my phone to show up in my networks, selected it, entered the password and connected without any issues at all. Once I was connected, in the Wireless Tether app, I went to went to menu > Access Control and toggled Enable on and then off. I was off and running! Same goes for the ipad. I had both the iMac and iPad connected at the same time and running with no issues at all.