Thanks for the info on metro passes and carnet. Will do as suggested (get both!).
Forgot to ask -- What is the most direct (easiest?) way to get from CDG with luggage to Gare d'Austerlitz? We are flying to Paris tomorrow. Thanks.

Most direct:
RER B to St-Michel Notre-Dame

Easiest:
Air France bus to Gare de Lyon
Taxi to Gare d'Austerlitz

(Remember: you will be getting on a train with luggage at Austerlitz station. The RER is just another train.)

<b>Correction</b>

Most direct:
RER B to St-Michel Notre-Dame
RER C to Gare d'Austerlitz

Robespierre,
You have been incredibly helpful -- thanks. The only thing I have left to do is get on the plane!

I know it's annoying (not to mention illegal), but some taxis might refuse to take you for the short ride between Gare de Lyon and Austerlitz (which is actually quite doable by foot, if your luggage is reasonable, via the Charles de Gaulle bridge). I thought I'd better warn you... Otherwise, yes, the RER B + C combination is a good one, but be prepared for long corridors at Austerlitz.

RATP buses 24, 57, 61, and 63 will get you there, too!