I am a doula and a midwife's assistant. I have a client who is currently on lithium and have been researching options for her. This is what I have found. The most comprehensive, readable source for info that you can get is Hale's "Medications and Mother's Milk". Make sure to get the 2002 edition. There is a chapter on lithium there that says essentially this:
Many health care providers say that breastfeeding while on lithium is absolutely contraindicated, however, each baby will metabolize lithium differently and some will show effects of lithium toxicity, including heart, thyroid and kidney problems. Most of these problems can be remedied if the baby is exposed to less lithium. Lithium does pass easily into breastmilk. If you choose to breastfeed while taking lithium, you will need to take the baby to have blood drawn to check for lithium levels frequently (every 1-4 weeks).
If you have not selected a medication yet, you may look at some of the alternatives to lithium. My research indicates that lithium is one of the safer choices in pregnancy itself (though not without risks) but that once the baby is born, carbamazepine or valproic acid may be safer.
(Disclaimer)I am not a physician, just someone capable of reading medical lit, so you will want to do your own research. Hale's book is a great place to start and is a good resource for any nursing mom.