I've tried it. At home I have a harmon/kardon go-play and a Bose SoundDock. It's nice to be able to use the iPhone 3G on these without having to charge separately.
With it attached to the iPhone it fits nicely on the harmon/kardon, but on the Bose SoundDock it stands up too far. I use one of the (CableJive) dock extender cables for that setup so the iPhone doesn't have to sit so far up on the Bose. Works nicely.