At the moment I don't think that it is (particularly) likely that 2009 will be the last year of the PS3 ... 2009 could change a lot of things though.
Right now it looks like there are a lot of publishers and developers who are willing to support the PS3 and XBox 360 heavily because their combined userbase justifies the development cost of the game. As long as they both continue to have strong enough sales that this continues I don't see any reason why third party support would waver. If there is a steady stream of games being released for a system people will still buy it and it will continue to "live" ...
How 2009 could change things is simple, we're likely going to see Nintendo or Sony announce their next handheld system (for release in 2009 or 2010), rumors will start to fly about a new XBox system (the 360 will be 4 years old), and the Wii will start seeing the third party support its sales have justified. If you combine this with people having less money to spend on everything, it is quite possible that the PS3 or XBox 360 could see lower sales and/or the Wii could see higher sales which could change the balance of the market for 2010 ... I'm not saying this is highly likely, but if the Wii passes 60% marketshare I don't think you're going to see much interest from publishers in producing the next $20+ Million blockbuster for the HD consoles.