I'd definately keep an eye on them if you let them roam freely together. Bunnies can pack quite a kick that could actually hurt your cat pretty severely. It will probably take a while for them to get used to each other. The bunny is going to view the cat as a predator. Eventually a lot of cats and rabbits get alone well, they'll actually play games at being the hunter and the hunted. They will groom and cuddle. Bunnies have even been known to pounce on sleeping cats and to play other tricks on them. If the bunny is freaked out by the cat try to put the cage up higher, maybe level with your waist. The rabbit will feel more secure this way. Also, never let the cat go into the rabbit cage. Rabbits are very terratorial and need their own private space, intrusion in this space causes them to act up or just be freaked out all the time. It will probably take a while, but rabbits and cats can be friends- but supervised friends only.
check out this website for helpful advice- http://www.rabbit.org/journal/2-11/c...d-rabbits.html
We adopted a rabbit from this organization, but she actually kept attacking and mounting our cat- no joke! We had to take her back to her foster home because she was miserable with us. My fiancee has had rabbits his entire life.