Thanx to all the Arduino Linux guys for describing their installation steps. I'm running a AMD64 Ubuntu 5.10 and noticed some small differences so I made a document discribing the steps I did to get Arduino up and running: go.yuri.at » Installing Arduino on Ubuntu 5.10 AMD64
@ICEM4N: could it be that you have another RXTX copy installed somewhere, see this similar error message from Processing: http://processing.org/faq/bugs.html#serial