Is it okay to let my step daughter call me mommy? ?

Okay, the basics - She is 4 about to be 5. Her father and I have been together since she was 2, but she thinks I have been there since she was born. I have never asked her to, because her mother was still involved in the past and I didn't want to cross any boundaries. I still haven't asked her too, but she says it often, and I correct her. I am afraid by telling her she can't call me that she may perceive it as I don't want to be a mommy to her. She is the only child we have and we have custody of her. Right now her mother sees her one weekend a month and as of August 2009 her mother will cease to ever see her because she chose to marry an incarcerated offender against the court orders ( he gets released in August). I am like a mother to her and myself I feel like she is my little baby girl. She is our world. I just don't know what is the right thing to do for her. Help me please. Anybody who had a step parent they felt like was there bio parent or a step in the same situation. She is what is important to me. I want her to know I love her and would do anything for her no matter what she calls me.