WARNING this is not for the faint-of-heart! It will break upon SDK revisions! Because it might result in more work for GoogleOHA Engineers they might discourage it! WARNING, THIS ONLY WORKS WITH ECLIPSE! If you look at the SetUP Ant task code you will find that it checks for an sdk.ant.build.revision number with a max value of 3 set at this time. The value is stored in the sdk.properties file at the root of the platform version folder.
Now, here is the trick. Change the number to the max which is 3. DO NOT STOP YET AS YOU ARE NOT DONE. Copy the complete set of ant_rules_r2.xml, ant_libs_rules_r2.xml, etc and change the copy names to use r3. Add the new obfuscate target making sure to add the other stuff such as Proguard config checking, etc. Now change that dex target by setting depends to include the obfuscate target you added after the compile target in that depends line.
Now, fire up Eclipse and compile all you want knowing that its being obfuscated automatically. Neat huh? FINAL WARNING, I HAVE NOT TESTED THIS WITH ANDROID LIBRARY PROJECTS YET!
I found it as I was looking at ways to decrease the ANT build script code I was writing and I really did not want to re-invent what Google/OHA were already doing with anttasks and android development tools. Its not the tools folks, despite what Joe Hewitt claims. Its the knowledge of hacking up the tools to do what you want!