If you have a Mac that's more than a couple years old, chances are it's a 32-bit Mac.  Unfortunately, Apple is only distributing a 64-bit version of Java 1.6, so you cannot immediately use Hadoop even if you have the latest software update.  Below are the steps that I used to get Hadoop running on my machine.  If you have a 64-bit Mac, you only need to change the default version of Java your computer is using.

First you need to download and untar SoyLatte.  I placed it in /System/Library/Frameworks/JavaVM.framework/Versions/soylatte16-i386-1.0.3/ mostly because that's where the other Java versions are installed.  Feel free to place this directory anywhere.

Next, you need to get your computer to use SoyLatte by default.  In your .cshrc (I use csh instead of bash - modify these accordingly if you use bash) add the following lines:

set path = (/System/Library/Frameworks/JavaVM.framework/Versions/soylatte16-i386-1.0.3/bin $path)

setenv JAVA_HOME /System/Library/Frameworks/JavaVM.framework/Versions/soylatte16-i386-1.0.3/

That's it if you want to develop for Hadoop on your computer.  If you want to run Hadoop on your machine for testing purposes, you need to also modify conf/hadoop-env.sh to contain the line:

export JAVA_HOME=/System/Library/Frameworks/JavaVM.framework/Versions/soylatte16-i386-1.0.3/

You should now be up and running!  Let me know if you have any questions.

Kurt <tadayuki AT cs>