Check in your warranty manual that came with the XLR. In there is a way to contact your regional rep (also known as the "zone rep"). This is who to start with if you are having problems through the dealership. The regional reps can get a lot done and did for me when I had a serious problem with a past (long time ago) Cadillac. Your dealership can also give you the regional rep's number (and sometimes that also sends a message).
Good luck. The only good news from your post is it seems everyone is sure where the problem is located.
Let me add a couple of items from my original append...First, check your lemon laws in your state. Most states have a specific amount of time that a car may be "in the shop" before you can take action. This may be helpful if the part does not come in soon.
Having said that, the Cadillac division is very conscious of the XLR quality image. It would surprise me if this is not taken care of very quickly once the regional rep is notified. Keep in mind Cadillac has pulled cars off the line because of single defects and these are apparently not sold - which is why some of the forum members have had their VINs changed prior to delivery.
The one exception to that is if there is a design flaw in something where the manufacturer would have to perform a redesign. That just takes time to do a redesign. Since every XLR owner is not reporting the problem, I suspect that a redesign is not necessary in this case (but I really don't know).
Anyway, the best bet is to contact the regional rep and study the lemon laws for your state (although I don't think you'll need to mention those). Also, please append any results that you get.
P.S. If you want to really scare yourself, look on the web for Audi A8 and BMW 745i problems. There are always new model year problems, but those two models, more than others, seem to be using customers as their beta testers (IMHO).