Hey guys - thanks for all the info.
Looks like I might get lucky... I called a GM Parts place this morning and they confirmed that GM no longer supplies the part, but also said that it could still be available from existing stock providing you can locate it. They gave me a part number of 25616098 and gave me a phone number to call.
When I called the number and gave her the part number, she said they have 22 in stock at a price of $208.46 each. The price sounds about right, but she was unable to confirm vehicle application - only the part number. I got the impression that this place was not necessarily open to the public, but was more of a dealer resource or something.
Now I just need to confirm that that part number is indeed for a headliner that work in my car, since it doesn't match up with any of the part numbers I have. Although the parts place confirmed that part number for a beige headliner with sunroof opening, I'd hate to fork out $200 and realize once it'* shipped that it won't work.
Will keep you all posted...