Two Model S "Beta Prototype" Signatures in Pearl White and Signature Red both finished assembly at the Fremont factory on Thursday, 3/15/2012. This is according to George Blankenship himself during a videotaped interview I conducted in the Santana Row store on Friday afternoon.
They are #43 (Pearl White) and #44 (Sig Red). Their VINs are:
Later that night, I asked the Model S (and X) Product Manager how many Betas they plan to build before they start producing Release Candidates. He said he knows but can't release that info to us at this time.