SD-
I don't know about sailing nav packages, but if you've seen the job that TomTom and other "car nav" packages can do on the Palm Treo and other "smartphone" devices...Hey, it fits in your pocket, runs all day on a small battery, and a 320x320 "half vga" screen isn't such a bad place to start.
Of course Treos are unsuitable for use on deck, because there's no lanyard attachment point.
But a nav package that could live on a computer below, and then either "dump a session" or integrate with bluetooth to the handheld on deck...that's just GOT to be coming on the market soon. It makes entirely too much sense, and would do many jobs very nicely.