A.C.
I, unfortunately, was married when I got pregnant with my daughter. He didn't waste any time taking off, but because we had to get divorced, I had very little choice in whether or not he would get visitation. He has seen her 5 times in 2009 and just once so far in 2010.
I don't know the right answer. I can just tell you that you are doing the right things by providing the most consistent life that you can. I believe in the family unit, but sometimes no daddy IS better than a crappy daddy.
My daughter turned three in March and she tells the most fantastical tales about stuff that happens at "my daddy's house". A house that she has NEVER been to nor seen any pictures of or knows anything about. I cried nearly all night the first time she got mad at me and started crying for daddy--this started more than a year ago!
BUT...then the other day she tells me that I am the Mommy AND the Daddy. I've never told her anything like this...nor anyone else to the best of my knowledge. She knows she has a daddy, but in the moment I am both. She has even started calling me daddy occasionally.
I guess what I am saying is that there is no one size fits all answer. It is good to be honest with our kids, but the age appropriate answer is "Daddy doesn't live with us." That will probably do for now. "Why" will definitely come later. If my daughter asks to see daddy, I tell her that it isn't our turn right now...she understands taking turns. If she wants to talk to him, I do my best to get him on the phone.
I've also been keeping a journal since her birth. It has the good, bad, and the ugly. It is mostly filled with milestones and funny things she says, but it also talks about how I am feeling and what is happening with her dad. I don't bad-mouth her father, but I tell what has happened in an objective way..."Your dad called and said he would come by today, but he didn't make it." I can only hope that when we get to the ugly teenage years that she'll understand that I did the best that I could with what I had.
I also tell myself all the time to remember that there are much worse things that your child could have to experience in life than just having one parent or having a mom that has to travel for work or whatever.
Good luck.