I would recommend LaCie's Pocket CDRW. I just got mine two days ago and it worked right out of the box with my Pismo and OS 9.1 without installing any drivers. I used Iomega's Predator (OK, it was only an USB drive, but you can get a FW version, too) for a while and had some hassle with it.
However, the best thing about Pocket is that it really is sturdy enough for serious mobile use. With Predator I was all the time fearing that I might accidentally break it in my bag. Also Pocket is smaller, although a little heavier.