I use a Minoura iH-100S for my Samsung Nexus S Android smartphone. I have done about 1000 km with it so far. I don't use any kind of cover for the phone, except if there is a drizzle, in which case I wrap the phone in cling film (like for wrapping food in the kitchen).

I've not had any problems so far. With the Minoura, the clamps that hold the phone in place snap apart when the release button gets pushed. To guard against accidental releases I wrap a rubber band around the clamps, just for peace of mind.