Yes, some bunnies do a teeth grinding thing that sounds a bit like a purr. My bunny never did it, but my friend had one that did. It's a sign of total bunny bliss. (Though, as stated before, they also grind their teeth when they're stressed.)
My bunny, Hunny Bunny (RIP
), was a free-running house rabbit, and we got Tailer while she was still with us. They were the best of friends...and Hunny was definitely the boss in that relationship. Before we got Tailer, we used to leave Hunny with my parents, who have two cats, when we went on vacation, and she got along great with her cats, too (though they were never left unsupervised). My friend with the purring bunny also had five cats, and they never threatened the rabbit either (though the rabbit did sometimes threaten the cats).
I'd just advise to, please, be really sure of what you're getting into before adopting a rabbit. I used to do volunteer work for the House Rabbit Society, and we got dozens and dozens of calls every week from people who wanted to give up their rabbits because they were more work than they had planned on. They're wonderful animals and definitely worth the effort, but they are significantly more work than cats.
A good resource for information about rabbits is the House Rabbit Society web site, http://www.rabbit.org/