Given the flood of screenshots from various outlets and languages, it is likely that the port has appeared on PartnerNet, a closed Xbox Live network only accessible through development kits and debug units.
PartnerNet availability would also suggest that the price point of 1337 Microsoft Points ($16.71), as specified in one of the shots, is placeholder. nope
Shacker ease one reports that the 83 MB Xbox Live Arcade port of Duke Nukem 3D is indeed downloadable through the closed PartnerNet network, though the current build only "displays a rotating poly and that is it."
Original Story: Two alleged screenshots showcasing the downloadable Xbox Live Arcade port of Duke Nukem 3D in the Xbox 360 dashboard have appeared courtesy Joystiq and tipper SV.
The enhanced XBLA release will arrive this year with co-op and online multiplayer in tow, according to a Dallas Business Journal report that contained "off the record" information.
While other details of that report--supposed platforms and release date for Duke Nukem Forever--were quickly removed, the Duke Nukem 3D port information remained intact, suggesting its validity.
Developer 3D Realms has previously indicated its desire to port the classic ass-kicking, bubblegum-chewing PC shooter to Xbox Live Arcade. At that time, the port's release was uncertain, and the company has yet to make any further official comments.