Poinsettias typically grown for American households for holiday decoration are not very toxic in small amounts. It is the non-hybrid and wild species that are highly toxic. Monitor your pet. If it quits eating, drinking, pooping, or is acting lethargic you *must* take it to the vet.
In the mean time, look at the House Rabbit Society's website (www.rabbit.org) for good information on rabbit care and poisonous plants.
Also consider getting your rabbit pet insurance. It is affordable and takes away worry when there is an emergency. Vet Pet Insurance is the one I use for my menagerie.
I hope this helps and I hope your bunny will be fine.