While the latest iteration of the 18" Alienware laptot is listed as "Alienware 18" or "New Alienware 18" on the Dell website, all order forms and shipping documents and support references list the actual model as "M18XR3". So to be clear: Dell is continuing with their current naming convention for their Alienware laptops.
The same is also true for the new 14" and 17" editions. So in essence:
The 2013 "New Alienware 18" is the M18X-R3 (or "M18XR3 Viking")
The 2013 "New Alienware 17" is the M17X-R5 (or "M17XR5")
The 2013 "New Alienware 14" is the M14X-R3 (or "M14XR3")
I have updated the Alienware Wikipedia Article to reflect this, and hopefully this can help clear up any confusion about the new models - especially when debating their specifications or when helping new owners.
You may verify this by completing a build on Dells site, adding it to your shopping cart, and then clicking the "view details" option of the shopping cart. This will give you the complete breakdown of all Dell products in the order, including the main Base of the machine. The base itself has the new model number, while a few minor components (like stickers, support material, etc.) are reused from previous generations and still carry the older model numbers.