Hmm. I've never done the work you are describing but I'll take a stab at it anyhow. I believe the 2005 NEC says that 3/0 copper should be the common grounding electrode conductor size for a multiple story building. You'll also need to use a split-bolt connector to tap off the common conductor to each sub-panel location. But I'm sure you already knew this.
If I am wrong about any of this, feel free to correct me.
