I'm using a SRAM rocket (similar to the older wrap round indicator X7 which is what I have for the front shifter) with an XT rear mech just fine and the wife has a SRAM MRX gripshift with LX rear mech, but as Nick says the Xx (where x is a number) won't work.
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This is what i have been told
All covered in the FAQs above.
Parktools :?:SheldonBrown