If they announce Forza 3, I doubt it will hit this year. We'll probably see it mid-2009, which would put development at two years.
Plus, Turn 10 has the advantage of already having the engine and much of the code necessary to speed up the process.
I'm excited at the thought of Forza 3 releasing at the same time as GT5. While I will buy them both, Forza will probably get more playtime. The online component in Forza 2 kicked serious ass. It's probably the best online console experience I've ever had. Great community, awesome features, avid fans, some of the most mature people I have seen online.
I guess the difficulty level of the game keeps the 12 year old shitheads away from the game.