hope i'm not imposing again here....
when i had my first DD her father was heavy into drugs and would rather play basketball than hold down a job to support his daughter.. my father was the closest thing to a "real" father as she had. until she was 4 when my dad passed away. After that she really didnt ask... when she did depending on her age, like when she was in elementary school she'd ask.. and i'd tell her "your daddy made some bad decisions".... and i stuck to that. as she got older and i had my 2nd DD she asked more about her (deadbeat) father. at that point she was 13 and i knew she could handle a little more of the truth.. so i asked her if she knew what drugs were (duh she's 13 not stupid) and she said yea (and rolled her eyes i might add.. lol) and i said her father started on drugs and got himself in trouble, and its hard for him to keep a job because of his addiction. that held her over until she was about 15 and started showing more like "him"... ugh. So i had to tell her the honest to God's truth about him. How he was violent, he has mental issues, the list goes on.
So i think it all depends on the age. I would never suggest lying to a child about where the sperm donor is, because they will resent you for that. Always be as honest as you can, never bad mouth the FOB cause that will make you look like the bad guy... and always let your LO's know how loved they are and how important they are to you.
just my 2 cents.. sorry to babble
oh and my oldest DD is the one in the strapless dress next to me (the bride) =) i was 17 when i had her and she is 16 now.