Now the funny thing is I have the same problem. I asked the mechanic at my garage and he reaconed the release arm bush was siezed as well. I took the slave cylinder off and operated the release arm by hand. The play in the release mechanism showed that the bush was definately not siezed. The slave cylinder is seen to distort on its bracket when the pedal is operated.
Now I would have come to the same conclusion as you Chaser in saying that the problem is inside the bell housing except for one thing. It only happens if the slave cylinder is hot. For example, if the air conditioning is on then its much worse due to the heat coming off the condenser. If the weather is hot its not good either. However, if the slave is hot and playing up and I send a stream of cold water onto it then the problem is cured in seconds.
To be honest the problem is not bad enough to warrant a box out job. So I'll put up with it until the clutch wears out then sort the thing properley.
To answer your question C13tay about why the car doesn't lurch at start up. The problem isn't the clutch isn't releasing at all, the problem is the clutch has not releasing completely. Therefore the friction plate drags slightly on the fly wheel / pressure plate. The lay shaft that runs from the clutch to the gearbox is still being spun, but with effort it can be stopped. Now the gearbox synchromesh employs something called 'Cones' which are effectively clutches in their own right. They engage as you slide the gear lever towards the gear you require so to 'synchronise' the input shaft to the output shaft so that the gears are rotating at the same speed to allow a non-crunch engagement as you complete the shift into gear. Now as the clutch is dragging, it has more friction than the cones can apply so the gears can not synchronise properley. There is a second mechanism which I admit that I do not understand at the moment which holds the gear from engaging until synchronism has occurred. Because synch can not be achieved then the gear won't engage. Now, if you put the car in gear and then start the engine then the wheels gripping the road is what the clutch has to fight against and it definately can not win over that. Hence you can then drop the clutch proper and drive away. Phew, I hope that makes sense.