Rather than do this, I Googled it.
First up, the AirgunBBS site where there's a discussion on the matter (here). This isn't terribly clear: it might be tapeworm, it might not; it might be O.K to eat it, it might kill you stone dead. Many shades of opinion.
I Googled a bit further and found this. Well, this is better: The cyst means that the rabbit is, indeed, carrying the larval stage of a tapeworm that can go on - if eaten raw - to reach maturity in the stomachs of dogs, coyotes and foxes. It doesn't grow in humans - and cooking kills it anyway. Bottom line: it's safe to cook and eat.
I cut the cyst off, chucked out the innards, stuck the rabbit in pressure-cooker, whacked the lid on and boiled the hell out of it for about an hour and a quarter. I was going - belt and braces, this, I know - to roast it after I'd boiled it, but seeing how it came out of the pot, though, I thought I wouldn't bother: I curried it and scoffed it.
Now, I am - by nature or nurture, whatever - a very squeamish person. Gutting rabbits? Every single time I do it I grit my teeth and wince with nauseated disgust. So seeing the cyst on the rabbit did indeed give me the willies and no mistake. But I know that I'm already the carrier of one bizarre, parasitic and debilitating organism: I carry the belief that food which hasn't come from a supermarket must, therefore, be worthy of profound suspicion: if it isn't from Tesco's it's bound to be deadly. I certainly share in this delusion and much of my hunting is a conscious attempt to refute this belief which I know - to some degree, anyway - I share. This nonsense lives and prospers within me and, seeing the cyst, it reared up and - for a while - took me over completely: What if I get sick? What if it's bad? Perhaps I'll die?
But no, bugger it: I'd sooner get a sodding tapeworm than be afraid to eat anything but Tesco ready-meals for the rest of my life.