After checking off the pre-reqs, the process was as simple as following the small set of very clear instructions on the Wiki page.
The only small side excursion I had was that my bash environment had a JAVA_HOME env var set, which aborted the build at one point. Unsetting that and restarting the build, it then ran flawlessly for about 30mins before out popped a shiny new Open JDK 7 build.
Having never manually installed a JDK on my Mac (Apple updates have served me well) the instructions for how and where to install it was very helpful. And so simple! Particularly with how it results in the new JDK being visible to the
sbutton:~ $ /usr/libexec/java_home -V
Matching Java Virtual Machines (4):
1.6.0_24-b07-334, x86_64: "Java SE 6" /System/Library/Java/JavaVirtualMachines/1.6.0.jdk/Contents/Home
1.6.0_24-b07-334, i386: "Java SE 6" /System/Library/Java/JavaVirtualMachines/1.6.0.jdk/Contents/Home
1.7.0, x86_64: "OpenJDK 7" /Users/sbutton/Library/Java/JavaVirtualMachines/1.7.0.jdk/Contents/Home
1.7.0, i386: "OpenJDK 7" /Users/sbutton/Library/Java/JavaVirtualMachines/1.7.0.jdk/Contents/Home
sbutton:~ $ export JAVA_HOME=`/usr/libexec/java_home -v 1.7`
sbutton:~ $ java -version
openjdk version "1.7.0-internal"
OpenJDK Runtime Environment (build 1.7.0-internal-sbutton_2011_06_16_11_39-b00)
OpenJDK 64-Bit Server VM (build 21.0-b15, mixed mode)
Now looking forward to trying out some of the new language features with NetBeans 7.0.
All in all, a total breeze! Kudos to the OpenJDK Mac porters group.