That's not a good idea, it is still a dairy product. Letting it sit in a sunny window all day is just asking for problems.
Here are a bunch of tips for quickly softening cheese cake for baking.
I'll list some of the best here, you can click the link to read the rest:
http://chowhound.chow.com/topics/343629
*cooks illustrated recommends wrapping the cream cheese or butter in plastic wrap, then submerging it in a bowl of hot tap water for 5-10 min.
*I take it out of the metal wrapping and microwave for 20 seconds for cream cheese.
*I usually cut it up into small cubes, spread out on a plate and let it sit for 10 minutes at room temp...you have softened little cubes of butter or cream cheese pretty quickly.