Wikipedia is my friend ..

At the beginning of tonight's (7/4/11 UK) show Ryan introduced "legend" Todd Rundgren .. whom, until she was 9, was thought to be the Dad of Liv Tyler, now known to be the daughter of Steven Tyler. Stony look from Steven!

Until that age Liv thought Todd Rundgren was her father because her mother had got together him when she was little and she didn't see her father regularly. Good for them for not confusing a small child with politically correct nonsense.

To clarify, Liv believed Todd to be her father when she was growing up, though Todd apparently always knew he wasn't the father.

Apparently Liv guessed her father's true identity herself when she saw photos of her mother with Steven Tyler from before she was born, and wondered why she looked more like Tyler than Todd!