SSRS (2005): conditionally format null/zero count cells based on group
I finally found the solution was to reference the category it's grouped by (weeks wait) - it's a little cumbersome because it's a string, but: 


  =Iif( Instr("7-8wks8-9wks[...]51-52 wks52 wks +",ReportItems!Weeks_Wait.Value) > 0,"Red","Green")