Well, we're not the only ones who think it would be cool. The guys at 3D Realms are well aware how great Duke Nukem 3D was ... in fact, they've been working on a sequel for some time*. Following the release of Doom on XBLA, the prospect was raised on the 3D Realms forums to which a patient staffer replied, "It's already been discussed here. Whether it happens or not I can't say. It's ultimately up to Microsoft as to whether they want it or not." He elaborates here and here.
Indeed, the XBLA approval process (as we know) can be pretty intense. The porting process and integrating Xbox Live is no cakewalk, and that's if the title even passes portfolio muster. With the big dog of early FPSs already on the platform, shouldn't Microsoft be looking elsewhere? Not to mention, we'd rather not have 3D Realms do anything to further delay Duke Nuke Forever. Let's just keep our eyes on the ball ...
*Nominee for understatement of the year award.