To get IE7 support, the OpenLayers.Format.JSON parser should be used. This leverages the native JSON.parse method where available. It does add extra weight to UTFGrid builds. However, as of this commit date, we lose large portions of US govt agencies when we exclude IE7.
I would think there would still be an error thrown here. Will put together a test to determine for sure, but I'd think that the default GeoJSON parser used by the protocol would choke on the UTFGrid json.
