In point of fact, we don't know that Klein was wrong about Clark's fertility - the identity and parentage of the baby in the last episode was left deliberately nebulous.
On the screen, yes. But one of the writers came on to IRC after the finale aired, as well as attended various L&C fan gatherings, and told us what the facts were for this particular universe -- the baby was a boy, and he was one of Lois and Clark's biological descendents from the future, thus providing proof that Dr. Klein was wrong about Clark being infertile. It's why Sam, in the finale, said "That's not right" when he studied Dr. Klein's data. All this was going to be revealed in the S5 opening arc, until ABC reneiged on their contract with WB after renewing the show, causing the "To Be Continued" which was originally captioned over the final scene to be removed just before the US air date.
Obviously there's always a case to be made for deleted scenes not being part of official canon, but the show's writers had never intended Dr. Klein to be correct about Clark's fertility.
Kathy
