Unfortunatley that's one of the limitations of the matrix in SSRS 05. Supposedly that's been fixed in 08 but I've not had a chance to play with it yet.
As a work around, what I do when this is a requirement I do my cross tabs in my SQL code for my dataset and then just use a table to display it. IT's somewhat clunky and makes drill downs tough, it works.
-Luke.
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