The 'parameters' property was added to the Flash Player 10.2 and AIR 2.6 runtimes (reference: http://help.adobe.com/en_US/FlashPlatform/reference/actionscript/3/fla sh/system/LoaderContext.html#parameters)
However, any attempt to set the parameters property has been throwing a ReferenceError (ReferenceError: Error #1056: Cannot create property parameters on flash.system.LoaderContext) in Flash CS5.5 with the FLA set to AIR 2.6. If I sent the FLA to 10.2, it does not throw the ReferenceError.
This is a bug that must have cropped up in the last four months, as I just compiled this code in December when this worked fine, but now throws the ReferenceError.
Any assistance would be greatly appreciated.
You might want to post on this over at the Flash Pro forum. The current AIR SDK is up to 3.2 (this thread gives details on how some users have updated their copies of CS 5.5 with AIR 3.2), however given that this worked 4 months ago for you I don't believe updating to 3.2 will solve your problem.
The next step, if you believe this is an AIR problem would be to create a new bug report and provide us with some sample code or project that will allow us to easily reproduce the issue. You can do that over at bugbase.adobe.com.