alternate way of driveing a highrez mesh with i lowres one is to duplicate the lv0 of the subD version and use it as a wrap deformer on the highrez one..
then you can hide the sudD and just animate the low poly verson..
makes animation lots faster

also this alows you to split your highrez model into smaller peices and asign them as wrap deformers individually.
this can save alot of memory if youre makeing morph targets for example.. (then you whould do them on the head only and not the whole body)

this can be done with the whole body too
this can also be done with a nurbs highrez but it takes a bit more work doing the lowrez poly version..

edit:
hm.. i explain like sh*t hope you get it.. if not, feel free to tell me and i'll explain it better