Japanese blog Sinobi reports that there are retail listings for a Hotel Dusk sequel for the Nintendo DS titled Last Window: Midnight Promise (Last Window: Mayonaka no Yakusoku).
According to the blog, CING's sequel is set in 1980s Los Angeles and will hit Japanese shelves January 14. A cursory search of Amazon Japan doesn't turn up anything, but Sinobi somehow has box art.
Nintendo doesn't comment on rumours or speculation. However, we can puzzle this out with some simple logic and a basic sense of economics: First, Hotel Dusk did pretty well for itself both in American ratings. Second, it did well for itself in Japanese sales. Conclusion: a sequel two years out would make perfect sense and it might even come Stateside.
Time will tell if we've solved the mystery.