I am going Liams and here's why
Making it Liams ties Steffy to him for-ever, thus causing continued angst for him and who-ever he is with. Lets remember that Bell has no real imagination, if he actually made the baby Bills he could have some brilliant storylines of angst between Bill/Steffy/Ridge/Taylor/Brooke/Wyatt, heck it could actually be brilliant--and that's why I have ruled it out. I will say that Bell goes the rinse and repeat storyline -(remember Sheila and Brooke). He will probably do something incredibly lame like having Bill decide he wants Steffy (and to take revenge on Liam) and demand a DNA test after the big reveal. As these labs are never locked and have absolutely no security (for wither embryo's or Dr's) in Bells world, he will switch the results. The only way this could prove interesting is if the baby is really his and he switches them in an attempt to " save his sons marriage". I can just feel something mega lame like this coming on. Personally I had hoped that she wouldn't be pregnant, and that if Bell was going to get her and Bill together that he would make a real love story out of it and that he wouldn't throw her under a bus - unfortunately that hope proved forlorn
Anyway, I thought I'd ask for people to call it, lets see who's right.