It's been a little minute... but there is no way that's not his baby. I give you examples to prove my case.
1. Michael and Sun develop and strange attraction in the first season.
2. Sun goes out of her way to stop him from fighting Jin in the first season (slapping him in the face).
3. They almost kiss in the missing pieces episode, "Buried Secrets".
4. When Sun tells him that she is pregnant on the Kahana in season four, he has a very particular reaction. It seems like he is certain of something that he will not mentithumb|300px|right|Wierder than LOST itself. on.
5. Why the hell would a mother so willingly leave her child in the hands of relatives that she has never loved or trusted? I'm going back to the island... raise my kid for me... I'll find his father and we'll be happy... everything will work out? Sun doesn't seem to care about her kid at all at this point. She just wants Jin back.
There are several others, but I'm too drunk to think of them.
Then there is this (by the by, the strangest lost promotion ever) http://www.youtube.com/watch?v=SLg7smJJU6s&feature=fvw
PS- What's up with Locke as the Conductor?