I have some USB devices connected to Max 6 on OS X Lion including a Behringer BCF and a couple of M-Audio keyboards.
If I disconnect and re-connect a device, it will stop working. The only way I can get the devices working again without restarting Max is to:
-disconnect device from USB
-send a bang to the midiinfo object
-re-connect the device
-send another bang to the midiinfo object
If I don't bang the midi info object when the device is disconnected, Max wont receive any MIDI information.