There are a bunch of schemes for routing the cables past the bottom bracket. Most modern bikes use a plastic guide that sits underneath the BB tube. A lot of older bikes would use a guide that clamped onto the downtube and routed the derailleur cables over the top of the BB tube. There are some bikes that have grooves designed right into the BB tube itself and the cables just sit in these grooves. I'm pretty sure that's the way Rivendell does it with some of their bikes.
In the end, the important thing to remember is that it is very important to make sure there is some lubrication that makes the cable slide easily around the bends. When cables get sticky on their way around the BB, then you will have shifting problems. This is a common problem and is addressed by Sheldon Brown in his oft quoted article on "Autoshifting".