Some rabbits learn to use a cat flap if it is a light one. Cat flaps are sometimes used to give privacy for breeding rabbits in laboratories (Beaumann et al., 2005). Watch a rabbit using one at http://www.youtube.com/watch?v=F21hkMMgRpM
NEVER stuff a reluctant rabbit through it – that will put them off for life. Try holding the cat flap open using wooden clothes pegs. Each time the rabbit goes through, you will have to change the pegs to the other side. Over time slowly lower the flap, making it only half open so that rabbit has to push with its head to open it.
Never let a rabbit out unsupervised into an unprotected garden if there is a chance of foxes, cats or even birds of prey.
Baumann, P., Oester, H. & Stauffacher, M. (2005), ‘The use of a cat-flap at the nest entrance to mimic natural conditions in the breeding of fattening rabbits (Oryctolagus cuniculus),’ Animal Welfare, 14, 135-142.