Nintendo revealed their fall release dates and there doesn't seem to be a window for the 3DS to fit into.
Nintendo revealed the release dates for what appears to be their game lineup for the rest of 2010 today. The release dates seem to cut down the chances of the 3DS coming out in 2010, which means it's even more likely that Nintendo's next handheld won't hit until 2011.
The premium release date of the Sunday before Thanksgiving (November 21), which in the past has been populated by a marquee title or a new system, is occupied by Donkey Kong Country Returns. The prior weeks in November are also booked solid, with PokePark, FlingSmash, and Mario vs. Donkey Kong: Mini-Land Mayhem releasing the three previous weeks.
Nintendo doesn't usually double up on releases on the same day, and the fact that all of November is more or less booked solid seems to make the odds of the 3DS launching in the month very unlikely. The only apparent chance we have of seeing the 3DS in 2010 would be if it comes out after Black Friday, which wouldn't make sense.
While I could very easily be proven wrong, all of this information seems to narrow down the North American release of the 3DS to Q1 2011. It might still come out in Japan this year, as their release schedule is currently unknown. For those who haven't kept score, the official word from Nintendo is that the 3DS is coming out before March 31, 2011, which is the end of the fiscal year.