Step one
Check your Java Preferences Application to make sure you have the latest Java Apple provides via software update installed.
Step two
Right click the Minecraft application, and select "Show Package Contents".
Go to "Contents" and open the Info.plist file. I am using xcode, so it looks like this:
JVMOption values stand for the JVM Heap size. Increasing it to min 1024M max 2048M will bring you performance benefits. Warning: Make sure you have at least 2G Memory :-)
Step three
Copy /System/Library/Frameworks/JavaVM.framework/Versions/A/Resources/MacOS/JavaApplicationStub to /Applications/Minecraft.app/Contents/MacOS/JavaApplicationStub replacing the Minecraft file.
Step four
Enjoy your 64bit minecraft. The warning about the "far" render distance needing a 64bit java is now gone.