Here is the House Rabbit Society page regarding wild bunnies. http://www.rabbit.org/faq/sections/orphan.html
Basically, you should leave the bunny alone, as its mother is probably nearby--unless you are positive that the baby has been orphaned and is not old enough to fend for itself, or if it has been injured. In either of those cases, you should get the bunny and put it in a quiet, safe place, and immediately call a wildlife rehabber. They are listed by state here. http://wildliferehabber.org/st_disp_list.php.
You could also try calling a vet, some of them will treat wildlife, or will know of area rehabbers.
If you leave the bunny alone, you will need to keep dogs and cats up to give the bunny a better chance at survival.