Iceman: I've got two Garmins, a III+ and a V. Garmin sells a bicycle mount for their smaller hand-held units and that is what I've used to mount my GPSs. Very secure when you get the bicycle mount properly attached.
So check the accessories listing for your unit to see if they have a bicycle mount available.
Only thing is... remember to dismount your GPS whenever you get off the bike to eat or for a bio-break or else your GPS will be traveling down the road, displaying information, for some light-handed, lilly-livered thief.
