see http://help.arcgis.com/en/webapi/javascript/arcgis/jshelp/#ags_proxy
"... The application creates requests that exceed 2048 characters. While there is no official maximum length for a URL some modern browsers have imposed limits. Using a proxy works around this issue by performing a POST request rather than a GET request. POST requests are not limited by the 2048 character maximum because the information is transferred in the header instead of the URL. Common situations where you may exceed the URL length are:
-Buffer complex polygons then use the buffered geometry with a query task.
-Specify a spatial reference using well-known text (wkt)..."