In Reference to [NIM072906 ArcMap does not respect WMS background transparency when the WMS layer is initially added to the map.]
This is a function of MapServer code. Currently as I see it, the WMS in ARC Map 10 is not repsecting OGC requirements. ARC 10 is reading the input Image format as either JPEG or PNG. If v10 is reason the JPEG background Transparency color option is not avialable it is greyed out.
If PNG is the input format, then Background color transparency is not greyed out.
In my opinon this should not be read by the software. The user should be able to turn ON or turn OFF this functionality. For most MapScript writers there are two color reserved for background space and no data values.
255 255 255
0 0 0
It seems the ESRI Dev team is making this decision for the user. Rather than having an option to determine background color and respected Transparency. I believe this to be a problem related to Arc10 being more compatible with ArcGIS Server rather than following MapServer OGC requirements.
This option was used and respected in 9.3 regardless of the Image Input Type. This option should be returned so the user can make the decison on background and color transparency rather than the software making the decision.