With the invaluable help of David Green, I was able to run Eclipse on openjdk 6 on my first gen MacBook Pro (you know, those left behind by Apple, running on a 32 bit CoreDuo processor... good to know that Apple supports the early adapters).

With this script that I adapted from David's blog I was able to start Eclipse 3.4 (Version: 3.4.2
Build id: M20090211-1700) with Landon Fullers openjdk 6 build:

export JAVA_HOME=/Developer/Java/openjdk6-b16-24_apr_2009-r1
export PATH=$JAVA_HOME/bin:$PATH

java -server -Djava.library.path=$HOME/bin/jnilib -Dswt.library.path=$HOME/bin/jnilib -Xms128m -Xmx768m \
    -XX:MaxPermSize=192m -Dosgi.requiredJavaVersion=1.5 -Dorg.eclipse.swt.internal.carbon.smallFonts \
    -cp /Applications/eclipse/Eclipse.app/Contents/MacOS/../../../plugins/org.eclipse.equinox.launcher_1.0.101.R34x_v20081125.jar \
    org.eclipse.equinox.launcher.Main -os macosx -ws carbon -arch x86 -showsplash \
    -launcher /Applications/eclipse/Eclipse.app/Contents/MacOS/eclipse -name Eclipse \
    --launcher.library /Applications/eclipse/Eclipse.app/Contents/MacOS/../../../plugins/org.eclipse.equinox.launcher.carbon.macosx_1.0.101.R34x_v20080731 \
    -startup /Applications/eclipse/Eclipse.app/Contents/MacOS/../../../plugins/org.eclipse.equinox.launcher_1.0.101.R34x_v20081125.jar \
    -launcher /Applications/eclipse/Eclipse.app/Contents/MacOS/eclipse \
    -keyring $HOME.eclipse_keyring -consoleLog -showlocation -vm $JAVA_HOME

The script misses the shebang, since my hosting provider thinks that I'm trying to execute some serverside exploit...
You'll have to extract a couple of shared libraries that are packaged in your Eclipse distribution:

jar xfv /Applications/eclipse/plugins/org.eclipse.swt.carbon.macosx_3.4.1.v3452b.jar
jar xfv /Applications/eclipse/plugins/org.eclipse.core.filesystem.macosx_*.jar os/macosx/liblocalfile_1_0_0.jnilib

You'll have to rename all those libraries to give them a *.dylib extension.

Finally I had to point /System/Library/Frameworks/JavaVM.framework/Versions/1.6/Home to the openjdk 6 directory:

sudo mv /System/Library/Frameworks/JavaVM.framework/Versions/1.6/Home /System/Library/Frameworks/JavaVM.framework/Versions/1.6/Home.old
sudo ln -s LOCATION_OF_OPENJDK /System/Library/Frameworks/JavaVM.framework/Versions/1.6/Home