I think that it is possible to see it as early as Q4 2010 and as late as Q4 2013 ...
Nintendo will have sold over 120 Million units by the end of 2009, and even Nintendo would admit that breaking the 200 Million mark before releasing a new console is unlikely (and probably not the best business strategy); it would be a risky move because you don't want interest in your console to drop before you release a successor. On top of that Nintendo will probably release a successor to the Wii in a similar timeframe and I suspect that they would want to release them with at least 1 year (probably 18 months or longer) between them; to a certain extent, ideal release dates for both the DS and Wii's successor is probably 2011 or 2012, and the question within Nintendo is (potentially) which system needs to be replaced sooner and when should it be released.
