I lost my AVR GCC installation on the upgrade to OS X Lion and I can't remember where I got the previous version from.
There's CrossPack and OSX-AVR binary packages, and you can compile the tool chain by hand to get the latest version (for example, see Option 3 at ladyada.net but you'll need to apply the latest gcc patches).
MacPorts can download, compile, and install AVR GCC 4.0.2, but since moving to Home Brew for other application installation I was disappointed to find there isn't an official AVR GCC formula yet, but as luck would have it Lars Immisch has written a set of formulas to install GCC 4.6.1, they haven't been pulled into the main Formula repository yet so the simplest way to install is:
git clone http://github.com/larsimmisch/homebrew-alt.git brew install homebrew-alt/avr/avr-libc.rb
If you don't have brew check its homepage for installation details (a one-liner)
avrdude is available in the standard formula set:
brew install avrdude --with-usb