I've been having some issues with the IdentifyTask returning results when it's executed using IE 7 or 8. The task runs flawlessly In IE9 and any other browser. I've logged a support call but wondered if anybody has run into this issue. It's almost like the task isn't even executed.
If you're identify operation returns a lot of data, IE 7/8 could be choking on it. Have you tried running an identify that returns only a couple of features? Does it work if you specify return geometry = false?
Not sure if this is the issue but I don't see where you've defined the identifyTask or identifyParams using var. IE has issues with missing vars along with trailing commas and missing semicolons that other browsers seem to ignore. Try adding var to these statements:
identifyTask = new esri.tasks.IdentifyTask(service);
identifyParams = new esri.tasks.IdentifyParameters();