I drink a protein shake after every workout and on rest days as well. Someone once made a good point to me that it is not only the days that you train when you require a shake but the days off require recovery as well. After all, that is the point of taking the "rest" day, right? When she told me this, the lightbulb went on in my head! To me, the same goes for after cardio. We all work hard in our workouts and I think that no matter what the workout, our bodies deserve some recovery.
For very tough cardio workouts, I also drink about 12 ounces of electrolyte replacement drink (like sports drink but the ore high tech version with the good stuff but no sugars). I usually fill a 24 ounce poland spring sports bottle with it and usually drink about half, swtiching off with water. If I am suffering a lot, I drink more. Sometimes I drink less. I figure my body tells me how much I really need.