Everything I've heard- it's a long process that requires the bio-family to agree to it. If you're caught doing it without permission, the children will likely be taken away.
If you're just fostering temporarily- really think about what's best for the baby. Unless you know for a fact that the biomom is pumping and maintaining supply, the baby will likely be put on formula after no longer in your care. Exclusively breastfed babies don't always transition well to formula or even bottles- making the transition back to the biofamily even harder for the baby. I also don't know what would happen if they get the baby back and report that the baby refuses to take bottles or formula. They might take it as transition difficulties, it may raise a red-flag that you were secretly breastfeeding and cause problems for you.
If you're fostering to adopt, I'd see fewer problems with it, but it also strikes me as more hurtful to have a baby that you're planning to adopt and have as your child permanently get taken away.
That said- as the PP pointed out, you can likely get away with it. I've seen foster parents who did.
I've seen suggestions of making half of the baby's feed formula so that way the baby will take formula and the bottle and hopefully the transition to all formula won't be so bad (although, for my baby, at half of each, he didn't have tummy troubles, but all formula was awful for him until we found the right formula). I'd definitely make sure that the baby is used to taking a bottle even if it's of expressed breast milk.
If you're doing visits with b-mom, you can bring bottles of expressed breastmilk (don't tell them that's in it, obviously) or, if you're doing half and half- make those the formula bottles for the day. Keep formula and bottles around the house so that if someone visits it looks like you're using formula.
Also- if you have bio kids you breastfed, be careful who you tell that you breastfed your kids. For my mom, the fact she breastfed me was taken as a red flag that she was secretly BFing the foster baby (she wasn't) and so he was taken away.
(Note: if you get permission to BF a foster baby, don't BF in front of the bio-family/bio-mom unless explicitly asked to by them. I know someone who got permission, then BF'd in front of hte bio mom and got the baby taken away. Being okay with the idea of someone else BFing your baby and actually being okay with seeing it are two different things)
Edited by sillysapling - 10/25/13 at 3:51pm