The Xbox 360 Inventory Locator is up and running. Or do as reader Markus says and call Target Guest Services:
If you call guest services sometime after midnight but before 8 AM (why before 8? Because that's when the stores open) go through the VRU to inquire about the availability of an item and they will ask from some of your personal info (name, address, phone number, email, etc) and then the SKU/Item number you'd like them to do an inventory search for. They will search the inventory database with in a certain radius of your home zip code and tell you what stores have them in their inventory as of the refresh from the prior night. I only had to use this method for 2 days before I got my 360 this morning.
Guest services 1-800-303-0308
premium xbox 360 207-21-0001
core xbox 360 207-21-0002