Back in the old ArcPlot days you could eliminate boundaries between polygons with the same attribute value. This was on the fly and required no dissolve. Is there an equivalent in ArcGIS 10.0 or 9.3.1?
I am not sure of any on-the-fly solutions, but to enable this effect without altering the actual data, group the required classes in the symbology tab, which is what you may already be doing.
There is also the ability to apply representations to your layers.
