Hiya,
The [comport] object that comes with Pd-0.38.4-extended-RC5 didn't work for me.
But there has been development done on [comport] since then. I got ardunio <-> pd to work with the current cvs version, one of the nightly builds is fine (http://autobuild.puredata.info/auto-build/2006-12-06/). I use tof's SimpleMessageSystem Arduino Playground - SimpleMessageSystem.
Cheers,
Alexandre