We told our agency and the social worker who did our homestudy. We didn't decide ahead of time whether or not we'd tell the birthmom, because it would depend on her personality and what the agency thought about telling her.
As it turned out, our son's birthmom didn't want to meet us or know anything about us (because then it would be harder for her to stick to her decision) so we didn't tell her, obviously.
Kalli,
I caution you to be careful about bf'ing a foster child. Most counties will not allow it at all. And if they do allow it, you have to get your caseworker's permission, the parents' permission, and probably a million other redtape-type things. We tried to get permission to b'feed infants in our care, but everyone we asked just said "eeeew. weird." and that was that.
In retrospect, it would be VERY difficult to b'feed a baby and then have it go home. As hard as it was to see our fosterbabies go home, it would have been much harder for THEM if they had to go from bf'ing with us to bottlefeeding with their parents.
If there's any chance that your foster child will go home, I suggest that you think very hard about bf'ing. Pumping and bottlefeeding, on the other hand, is a great idea. As long as you mix it with formula so that their systems don't get messed up if they go back home to 100% formula.