again to go back to the original question that subflava posted. it seems that you're having a bit of difficulty grasping the concept that multiple devices can share the bus concurrently. now, before we get into arguements about concurrency, multiplexing, synch v asynch, burst, consider the following analogy.
i am running a train depot where we load shipments onto a train (the bus). let's say the train is moving at a speed (conceptually, it's clock rate) such that 4 empty traincars pass by my loading dock every minute. my job is to load cement bags onto the traincars. the best i can do is 40 bags per forklift run every minute. so what happens, every minute i load 40 bags onto one boxcar. i'm going as fast as i can, but still, 3 cars go empty in the time it takes me to load my 40 bags. the boss, seeing the waste, sets up 3 other loading areas, each with their own person/forklift. now we have 4 people going full speed loading 4x40=160 bags per minute.
you get the idea now4 40Mb/s devices on a U160 chain, blah, blah...
what you should check out is pcguide.com or techfest.com and read about bus architecture in general, whether it's pci,scsi, ide
p.s. is this multiplexing? it can be thought of as such conceptually-multiple information streams carried over a single line or set of lines.