Both pine and cedar shavings are are toxic to rabbits.  There have been many cases where they cause problems both in the respiratory system and in the liver.

If you want to read up on them, here is a good place to start:
http://homepage.mac.com/mattocks/morfz/rabcare.html#litterwarss
Here is an exerpt from one of the articles they link to:

Rabbits And Pine
In 1989, the House Rabbit Society, which has fostered thousands of rabbits until homes can be found for them, made an accidental discovery (Harriman 8-9). When a young, healthy rabbit died following a routine spay, HRS ran blood work and discovered that the rabbit’s liver enzymes were elevated far above normal. Because there was no obvious reason for this elevation, blood chemistries were run on rabbits in foster homes around California, many of whom turned up with the same enzymes elevated. The only difference noted in the rabbits’ environments was that those with elevated liver enzymes were all using pine shavings in their litter boxes or cage trays; the ones with normal livers were using cat litter.

In order to determine if the pine shavings were indeed at fault, the shavings were replaced with cat litter and the rabbits’ blood levels were rechecked a month later. Without exception, all the previously abnormal liver enzymes had returned to normal. At the same time, an informal survey was conducted of HRS members whose rabbits had died of liver disease (diagnosed upon autopsy or through blood work). Invariably, either cedar or pine shavings had been used with all of these rabbits.
(from  http://www.afrma.org/rminfo2a.htm  )

Please consider using something else than softwood shavings for your rabbit’s litter. There are manu acceptable kinds available, including litter made from recycled newspaper, and wood pellets, similar to the ines used in wood burning stoves. They are often also made of pine, yes, but they are heat treated (cooked) so the poisonous toxins and resins have been removed. Many rabbit owners also just use a layer of newspaper. topped with a layer of hay.
Also, do stay away from cat litter, it can be dangerous to rabbits!

From: http://www.rabbit.org/chapters/san-diego/behavior/litter_compare.html:
Clay cat litter: Although inexpensive, clay litter can be dusty and may encourage digging in the litterbox. There are also more absorbent litters on the market. Also, if bunny should ingest this litter, it could be fatal.

DO NOT USE clumping cat litters with rabbits. This litter clumps when exposed to moisture, and it can’t tell the difference between external and internal moisture. Rabbits tend to nibble on everything, and should they ingest clumping litter, they risk a potentially deadly intestinal blockage.

This site has a very good evaluation of many different litters.

Personally we use the wood pellets They are very easy and clean to use, and absorb urine and smell very well. They can also be composted and used in the garden, which we do.

Most of the foster homes in The Rabbit Haven system use either wood stove pellets or horse stall bedding as litter.  My understanding is that the shavings out gas a chemical (phenols maybe?) that is dangerous.  The pellets do not contain this dangerous chemical.

For my buns I use wood stove pellets if they are available (low supply this winter) or Cat Country.

Hazel-mom (wow – great info!_ and Osprey have shared some very helpful advice. I’ll echo that pine shavings are not a good choice as a bunny litter.