Hi Michael and eric,
I'm using Rhino 5.0 WIP 90-day Eval 2010-12-14 (to test RhinoBIM) 64 bit and I have installed also Rhino 4 SR8 eval.
No problem in copy/paste from Rhino5 to Rhino4 or MOI v2 and no problem in copy/paste from MOI or Rhino4 to Rhino 5.
In Rhino5 there is at least a new entity called extrusion (used in RhinoBIM for steel beam members); extrusion is converted in polysurface in copy/paste from RH5 to RH4 and in solid in copy/paste from RH5 to MOI v2.

Giovanni