Originally Posted by USAFAN

I have seen rush hour traffic until 10:00AM while driving to CDG (on the other side of the Autoroute and Peripherique)

I guess your best bet is RER to Denfert Rochereau and Metro (or Taxi) to Montparnasse (have done it, there are some stairs at Denf. Roch).

Roberto mentioned already the ticket buying circus at CDG. Most of the ticket automates are not working or don't take you US credit card, and the lines are long. I have heard that US American Express Cards are working .. may be someone can confirm this.
Your schedule is really tight, can't you take a train a little later, take the Metro to 'Edgar Quinet', one stop before Montparnasse, only 150 yards walk, and have a café at one of the outdoor cafes...