According to a pamphlet being handed out at the Tokyo Game Show by SNK Playmore, a version of Metal Slug is being brought to Xbox Live Arcade. From the look of the art at SNK's booth this may be Metal Slug 1 coming to XBLA. The text on the pamphlet translates to "the original."
SNK Playmore does not list a release date in the pamphlet for the Metal Slug title. Given the sketchy history Microsoft's had with Xbox Live Arcade titles of late (hello, Lumines) -- we're betting Metal Slug will appear on Xbox Live Arcade in 2007.