Manufacturers GENERALLY release new models after Labor Day. Nissan usually does it earlier than that, and as soon as the 2006s come they put large incentives on 2005 models. GM, Ford and Chrysler already have 0% financing in place all year long on most of their models, but they may also add additional (cash back or price reduction) incentives. Toyota has Red Tag Days on now, Honda will be doing their Summer Checkered Flag promo by June or July.
The deals you can get on 2005 models will depend on supply and demand. Generally if there is a ton of 2005 product left that manufacturer will be more aggressive on programs and the dealer will be more willing to wheel and deal as far as discounts go. So August is usually a good time to buy as the new cars are just around the corner and everybody wants to move out their 2005 stock.