If you want to use SUN’s java version with rxtx on any debian derivative system you’d be surprised to discover that jre can’t find rxtx libraries if you installed them using librxtx-java package. It’s because SUN does not adhere to debian naming and placing policies and looks for libraries in wrong place. Quick fix:

cd /usr/lib/jvm/java-6-sun/jre/lib/
cp /usr/share/java/RXTXcomm.jar ./
cd i386
cp /usr/lib/jni/lib* ./

That will make those libs available to SUN’s java and everything should work from now on.