There is another round of weekend closures coming up, of the RER B line between Paris and CDG, due to maintenance work:
March 26 - 27, April 2 -3 and April 9 - 10 (2011)
There will be a replacement bus service between Mitry-Claye and the CDG terminals. Allow an extra 30 - 40 minutes for the journey.
http://www.ratp.fr/informer/trafic/travaux_ratp.php