In FC4 (and I'm sure others will follow) if you have a Garmin unit with
USB you must either treat it as a serial device or (my preference) rmmod
garmin_gps before GPSBabel will work with it. There's probably some
modules.conf like subtance somewhere that can be tweaked to make it keep
its hands off my hardware.
(Apparently, the whole "hardware support doesn't belong in the kernel if
it can be done in user space" thing got lost somewhere. )
I suppose this might plague any user of a sufficiently new Linux kernel
and not just FC4.