I think Twesterm is right, but I also think there's a chance we'll see more JRPGs after Final Fantasy hits, for a number of reasons.
1.) Companies may be waiting to announce and hype JRPGs in the pipeline until the FF hype dies down, so more may appear afterwards.
2.) FF XIII may get some adults who were big Final Fantasy fans to buy into this console generation, and most of those FF fans will have other games like Grandia or Breath of Fire or Lufia that they remember fondly. It's not going to be a huge change numbers-wise, but this does mark the first major inducement those types of gamers have had this generation. (Well, unless they're also a Tales fan.) They may add to a vocal minority in the US.
3.) It doesn't have to convert a huge American audience in order to spawn more JRPGs in the future -- some companies are going to be making their decision based on the Japanese market, in which there will be a big gaping RPG-shaped hole after FF releases.
Point being, you probably won't see Americans significantly more excited about JRPGs, but we might end up getting more JRPGs anyway... which is what I care about, anyway 
The question isn't whether or not making JRPGs is a risk (it's always been kind of a thankless job to make a fifty hour story when you could make a seven hour one) but whether or not the people who want to make JRPGs in Japan will get the backing they need after Final Fantasy XIII hits. If the game does well, I think it's possible some publishers will be more willing to take the chance. I wouldn't bet on it, but I wouldn't bet against it either.
