There's a theory floating around that the Jeyne that Jaime meets in Feast is an imposter. This based on Cat's description of Robb's wife as having "good hips," which she does twice in aSoS, and Jaime's description of the girl as having "narrow hips." Not much to go on, but you can read more here, if you choose:
http://towerofthehand.com/blog/2010/...rth/index.html
Also, maybe they're playing it a little different on the show. In the books it's clear that Sybell Westerling is working for/with Tywin Lannister. There are still some out there who think Talisa will reveal that she is really Jeyne Westerling in disguise. Maybe she's lying about being pregnant?
Or maybe they're gonna kill her and the baby in the show instead of leaving her in Riverrun?
Or maybe the show is tipping Martin's hand. Can't WAIT to see how this plays out.