Dairy products are a rich source of calcium, and lactose, the sugar contained in milk, facilitates calcium absorption. However, chocolate milk is not a good source of calcium. Because chocolate contains calcium-binding oxalates, it can interfere with calcium absorption.
http://www.askdrsears.com/html/4/T040600.asp
A couple of other sources, however indicate that even if this is the case that the amount of oxalates is rather inconsequential... which is irrelevant because you're looking for protein rather than minerals.
I suppose I'd lean towards eggs, but you're looking at eating four eggs to get the same weight in protein that you'd get in a scoop of whey - which means you're looking at more than a couple dozen eggs every week (if you hit four each morning).