I have the D1x and my photofinisher prints files made at the JPeg fine mode at 9x12 without any pixelation. I cant imagine going up by two inches would make a difference.
They use a "Noritsu" machine the takes the digital file and exposes either Fugi or Kodak pager.
I have an Epson 1270 and have printed 13x19 prints direct from the camera both with and without interpolation.
My biggest concern and the reason I waited to get a digital camera was I said I would make the jump when I could get an 11x14 out of a digital that equaled or surpassed a 35mm negative. That time has long passed and I am excited by the prints I get.
I have cropped images by half and then used photoshop to interpolate them back up to 13x19 and have to say that with the correct interpolation action, if you start with a high enough res file, you should not see a drop off in quality.
In fact, there is something about digital files (if done right) that makes them have a certain "pop" if you ask me. The guy that runs my camera store claims it is based upon some subliminal ability of our eyes to "see" the pixels captured by the camera and reproduced on paper compared to the "smearing" of light that he says occurs when the light goes through a negative. While I am not sure I beleive him on this point, I do know that my digital files seem to have a certain crispness that I didn't get with film (except with chromes)
Maybe I am crazy, but I swear when i see a photo posted on the web I can tell the difference between whether it was a digital capture or a negative capture most of the time.
Am I a total digital nut....no. I will take some provia, or Tri-x or an entire list of films/chromes all day long under certain circumstances. But for studio shots of models, I wouldn't go back to film to save my life.