If I leave it in the pan in the fridge for too long The acid in the dressing will actually start cooking the breasts? Is this also true with fish and lemon juice?

It doesnt technicaly "cook" it, but it will affect the texture similarly to cooking. For exemple if you make a fish ceviche, you marinate the fish overnight in citrus juice... it will look kinda cooked, but it's different.

Thx

I LOVE marinating my chicken in Italian Dressing

Hmm, might try this... How long do you guys recommend?

Gross, make a real marinade!

Don't marinate in salad dressing unless it will stay emulsified. You are better off marinating in buttermilk. Salad dressing will leak too much acid into the meat if the oil and vinegar seperate, this will make the meat too tangy. The mild acidity of buttermilk is incorporated nicely with the fat and will result in a nice even marinade. (The acid is good because it breaks down the meat and makes it tender, but too much acid will make your chicken sour).