I have a problem connecting to my arduino (duemilanove 328) at 230400 baud - 115200 works fine. I use processing and rxtx to connect my arduino. I know that the maximal serial speed of macosx is 230400.
I just compiled the cvs head of the rxtx library myself (http://www.neophob.com/2010/07/serial-library-rxtx-v2-2pre3/ if you want it).
Did someone successfully connect his arduino at 230400 baud?