I know that this thread really hates speculation, but I really think that Talisa is a spy. Perhaps the very spy that sent the letter that Arya stole. Think about it. She's obviously attractive and performing a service that would appeal to the honorable Robb Stark. And she's from a far away place that would make checking her identity impossible. I don't know, I just get that feeling.
Hmm.. I'm not sure I get that from her. I feel like her function is to highlight honor and promises. Robb's obviously going to be stricken by her and want her quite a bit, but he'll be torn and told he needs to hold up his end of the deal.