In reply to Tyler:
Chains can break (doesn't seem to happen that often but it can) and maybe it's not that worn and that's why the shop is just saying they'll put it back together again.
Cassettes tend to wear with the chain, so it does seem the normal advice that you replace them together, some people will say the chainset too (or chainset rings if they replaceable separately). In the past I have replaced the cassette and chain, and other times I've only changed the chain. It hasn't seemed to have made that massive a difference either way - perhaps it depends on how perfect you expect the shifting on your bike to be! If you are racing, fair enough, it should be perfect but for most of us going to work by bike or out for some exercise at the weekend, a bit of clunk clunk and the occasional curse when it doesn't shift probably isn't the end of the world and it settles down as the new bits wear in.
If your drive train does get really worn though it can shift unexpectedly, and that can literally throw you if you are, say, standing up and powering through traffic it is not good. I got the biggest bruise I've ever had once on my inside thigh after a mis shift like that threw me forward and the bike's stem went into my thigh. I fell off, but fortunately wasn't in traffic at the time!