Cleese, Palin and the Welsh git are all signed, Idle knows about it but has not yet committed. The source article, Entertainment Weekly, does not mention Gilliam, but as they do mention Robin Williams voicing an animated dog, presumably Gilliam will be involved.
http://entertainment.slashdot.org/story/12/01/28/1340231/monty-python-crew-to-reunite-for-movie