Resurrecting this old thread...
I've been told the $1500 private offer is legitimate and can be added (that is, an additional $1500 subtracted) to the best deal you can arrange with the dealer. So go ahead and work your best deal, then pull out the private offer and they subtract another $1500.
It appears there are many ways to obtain private offer - once in a while GM sends them in the mail, or more recently the offers pop up when you search and scroll around on the GMC and Chevrolet web site.
A friend just purchased a brand new Sierra SLT 1500, 6.2L, 8-speed with max towing package - the list price was approximately $56k and he paid $44.5k. Apparently the dealer originally came down from $56k to $46k and then the buyer pulled out the private offer to get the final price down to $44.5k.
Anyone else heard about these mysterious private offers?