Know this is an old thread but the title is popular with people with a similar issue and there are lots of them it seems.
What worked for me is to look in the /Library/Extensions/Java/Extensions I found a file RXTXcomm.jar in there (from 2006) I trashed it and re-started Arduino... success! took me many hours to find it. You may have to show hidden files in the finder which requires Terminal and the following command (defaults write com.apple.finder AppleShowAllFiles TRUE) when done replace TRUE with FALSE and you're back to normal.
Since your original post come up to the top of Google search I hope this works for others too.