My ex and I both claimed my daughter as a dependent on our tax returns.?

I did mine and my husband's tax return, jointly, in January, claiming my daughter, since she has been living with me her whole life. We have already received our refund. I received a call last night from my ex (my daughter's biological father) telling me that he attempted to file his taxes with a status of Head of Household and claiming my daughter as his dependent. She has not stayed with him one day out of this year, but he does pay child support, not court ordered. He is stating that because he pays child support and because he was deployed all of last year that he should be the one to claim her and not me. I'm uming he is trying to use the argument that because he was deployed, that is the reason she lives with me, however, she has not spent one night with him since the day she was born (3 years ago), he has visited her maybe 4 times in 2008 and she has never even visited his home address, so she obviously does not share an address with him either. He is threatening to take me to court because his "accountant" has told him that he is the one that should be claiming her and not me. I can't imagine that this is even half true considering the situation, however, it does kind of have me worried. He told me that i will be getting a letter from the IRS soon and that I will have to amend my return. Is this true and if so, will I be audited and what will i need to have to prove that my husband and I are the ones who take care of her and is there any way for him to get around the definition of a dependent?