We;ve had that problem, never fully recovered and we ended up getting a new mattress. But now after 1 year of nursing, and other baby fluids, I feel like it smells and makes me smell too!
We steam clean ours and use Natures Miracle, which is actually for pet stains and odors. It's enzymes that supposedly eat away and take the smell out naturally. We buy it at Petsmart.
Also, we try to remember to keep a plastic protector underneath the baby and I. The problem with that, is it makes me very hot and sticky even with a sheet over it, so I often move it.
Good luck! There's nothing like the smell of sour milk in the morning!