At the Electronics Entertainment Expo 2006 last May in Los Angeles, Sony announced that its forthcoming PlayStation 3 console would debut worldwide on November 17 of this year. The company also revealed the console's two price points. Nintendo, on the other hand, was much vaguer about its new generation Wii console, choosing only to say that it would be less expensive than competitors and would arrive around the globe sometime in the "fourth quarter."

Despite what the Big N has said publicly, publishers and developers have prepping many of their Wii titles for an October release. This in of itself is not proof that Wii will also debut in October, but the data certainly doesn't conflict with the argument.
On top of everything else, this week industry sources have relayed that - in something of a surprise attack -- "Nintendo is planning to launch Wii before PlayStation 3." The latest word jives with publisher release calendars, whose Wii titles are scheduled in many cases for a mid-to-late October debut or, in some cases, very early November
http://news4gamers.com/ps3/News-1569.aspx

would be interesting to see how the sales of Wii and PS3 would go if Nintendo get their system out first particularly in Japan.

Personaly i think they are after different audiences but I think Sony may be slightly worried about the Wii beating it by a couple of weeks and stealing its thunder