I came across an issue with an article in the ArcGIS resource section. The article on ArcGIS Server and Customizing a WMS GetFeatureInfo response in terms of GeoJSON with the supplied featureinfo_application_geojson.xsl. I believe this is still and issue with 10.1
In short, to transform the WMS GetFeatureInfo response into GeoJSON (which is getting popular for good JavaScript reasons) ESRI recommend using a file called featureinfo_application_geojson.xsl supplied with ArcGIS Server. The problem is that this does not produce valid GeoJSON. The issue is that they put a comma at the end of each list, which means common JSON parser's such as JSON.parse() do not work.
SOLUTION: I fixed this with the following small changes to the supplied featureinfo_application_geojson.xsl.
RECOMMENDATION: Geoserver now does info_format = application/geojson, it would be cool if ArcGIS server could also do this. Not hugely important, XSL transformations do give flexibility.