Loose Cassette (SLR / X0)
I have a Mavic SLR wheelset, with an X0 cassette. The cassette is loose (rocking back and forth) on the splines, no matter how hard it's tightened.
My LBS suggested there's a missing ring, which should sit between the casstte and the wheel. Is this correct?
I'm not sure which particular wheelset you have, but on the newer road 700c wheelsets the freehubs are made wider for cross compatibility mainly between Shimano and Campagnolo 10 speed cassettes. It is also supplied with two spacer rings to pad out a smaller width cassette (usually for Shimano or Shimano compatible, ie SRAM, cassettes).
So, yes, I think that your LBS is correct.