A year after this post started (2004) I used AutoCAD and 3D solids to coordinate a Central Plant Electrical Power, 27 bus duct runs and only one part needed to be reordered. We were the only trade to do so in 3D. Today every trade has excellent tools in AutoCAD MEP Except electrical! No Busduct No Conduit. Nothing! Sorry it has conduit but not really correct conduit. Try to prefab off the conduit catalog and it all goes to hell. No correct bending radius or even OD sizes.
Autodesk Time to make this happen. Or at least buy into G.E.'s Autobus solution!
If you want to draw correct busduct there is a tool called Autobus buy G.E.. Everything you ever wanted. Works inside AutoCAD 2000.
Otherwise use the hvac tool for duct. Rectangular duct, size per your spec. and use 90 elbows and 45 elbows and you can limp through with minimal problems.