OK, I know I'm a bit behind the times...I upgraded ArcGIS Server from 9.3 to 9.3.1, and I'm using the 1.2 version of the Flex API. My IdentifyTask request now fails every time with a 500 error code and a message: "Unable to perform identify. Please check your parameters.".
I ran the app in debug mode through FlexBuilder and took the identify parameter values and entered them in the "identify" web page of my map services; that query worked, so I know it's not a problem with the map service. In addition, I have another app based on the SFV, and the Identify widget has stopped working there, also.
Has anybody seen this before? Do I need to upgrade to the Flex 1.3 API?