The official word is that this image shows a new Cadillac concept vehicle, but inside sources suggest this is the brand's upcoming flagship, codenamed XTS.

The e-mail from Cadillac arrived early this morning and just a bit ahead of the holidays, containing two gifts for fans of the long-time luxury leader.

The big news is the photo you see here, a partial image of what appears to be the long-rumored XTS, which will make its debut, in concept form, at the upcoming North American International Auto Show, in Detroit.  If it goes into production the new, rear-drive full-size sedan would replace two of the weaker entries in Caddy’s current line-up, the STS and DTS, and become Cadillac’s new flagship model.

Meanwhile, the terse release we received also confirms news TheDetroitBureau.com broke a few weeks back: Cadillac will add a high-performance version of its current 2-door base model, to be dubbed the CTSv.

We’ve gleaned a bit about the concept vehicle which was being called XTS inside GM circles.  It’s likely to share the Sigma platform that was used on the STS, as well as the smaller CTS sedan.  That means pure rear-drive for all U.S. Cadillac passenger cars.  (The outgoing DTS was a front-driver.)

This is an early spy shot of the CTSv Coupe, which will make its formal debut in Detroit, next month.

As the picture of the new concept suggests, Caddy designers are taking their cues from the enormously popular Sixteen supercar concept the luxury maker showed earlier in the decade, though we’re not talking something the size nor cost of a Rolls-Royce here.  The dimensions should be just slightly larger than that of the STS.

The critical challenge is to not only the STS-replacement from the smaller CTS but also make it look a lot more elegant and exclusive.   Though the new model will pick up on the basic visual cues of Caddy’s Art & Science design theme, the concept appears to be less angular and edgy than the CTS, which befits a more premium product.  But it has enough character to avoid the blandness of the current STS.

We’re betting the XTS, in production trim, would likely be built on the same Lansing, Michigan assembly line as the CTS and current STS.  Of course, that depends on the automaker committing the new concept to production, but considering the lackluster sales for both STS and DTS, we’re convinced it has no choice.

While we’ll have to wait for the Detroit Auto Show to get a look at the interior.  But if the latest version of the CTS is any indication, the XTS concept will bring significant upgrades to the interior of the current STS, which was marred by the heavy-handed use of cheap black plastic.  Look for something that could wind up taking on Audi for the title of most well-executed cabin, a GM source boasts.

As for the CTSv Coupe, if it is anything like the CTSv it will be equipped with a supercharged, Corvette-derived LSA 6.2-liter V-8, which makes a neck-snapping 556 horsepower and 551 lb-feet of torque.

As we reported last month, with the sedan version, both the automatic and stick pull 0 to 60 times of just 3.9 seconds, and will make 12 seconds in the quarter-mile. Top speeds are rated at 191 mph for the manual, 175 for the automatic.  A bit shorter and lighter than the 4-door, we wouldn’t be surprised if the CTSv Coupe clocks even better numbers.

The Coupe – both in base and v form – is not expected to be a high-volume offering, according to Cadillac General Manager Bryan Nesbitt, but it is nonetheless a critical offering in the luxury segment, where 2-doors serve as a sort of brand statement.

“This helps get us into the conversation,” Nesbitt told TheDetroitBureau.com during a recent interview, “and helps better position the brand.”

The Coupe v is set to go into production next Summer.  As Caddy is only showing its concept, for now, and not even confirming the XTS designation, we have no hard production date, though we wouldn’t be surprised to see it arrive sometime in 2011, most likely wearing a 2012 designation.

Expect to see both CTS Coupe and XTS earmarked for overseas markets, as Caddy aims to start playing again with the luxury big boys.  The maker has long struggled to get traction in Europe and these models could help.  But expect General Motors’ flagship brand to use the new models to make a big push in China, now the world’s largest auto market.