Possibly my problem is the same with many others who asked for help here:
I can connect my Atmega328 Duemilanove to my MacBook and it turns up alright upon ls /dev/usb, I can choose the cu.usbserial from the Tools-menu. BUt after that all that happens is the RX LED blinking and obviously no upload taking place. Th e TX LED doesnt come on. It works fine from a Win PC with the latest Arduino software and exactly the same settings.