When I had a 6mp camera, I found that 12x18/13x19 was as far as I was willing to go, and that was pushing it for some images. I've done some really nice 16x24's with the D2x, but probably won't go any larger for most landscape images unless I were stitching multiple shots together. 175ppi before any interpolation is pretty much my limit now, and 200ppi is noticeably better.
But the limit for me and my images may be different from somebody else. It really depends on a lot of factors, some of which are highly subjective.
1) Equipment and technique: using top-quality glass, and shooting from a tripod with cable release and MLU will give you a much better source image than a handheld grabshot with a consumer level kit lens. The former will be able to withstand much more enargement.
2) Image type: landscapes or other images with lots and lots of fine detail won't be able to to print as large before image quality deteriorates. On the other hand portraits or images that are more about geometric shapes than fine detail and texture can be printed at significantly larger sizes.
3) Post processing: artifacts from poorly executed sharpening, NR, tonal manipulations, etc will be much more noticeable once you print large. You can get away with being a little sloppy in your post-processing when you're printing an 8x10, but when printing at 20x30 you need to be much more careful.
4) Viewing distance: some prints are meant to be viewed from a distance. For instance a 20x30 headshot isn't something you'd probably want to view from a few inches away, it's meant to be viewed from a distance. But in my experience, large landscape prints are often viewed more closely. Sure, people will initially view a landscape from a distance that allows seeing the whole image, but then they'll often step up for a closer look; and if the image is heavily interpolated that closer look might not be so pretty.
5) Personal/subjective standards of image quality. Some people are pickier than others about what constitutes excellent print quality. The initial impact of a really large print can be pretty powerful to the point where you might not notice some of the flaws at first; but somebody who really takes the time to look might be a bit more discerning. When I hear people talk about a 6mp landscape image looking awesome when printed at 20x30, I have to wonder if they've ever seen a similarly-sized enlargement from a 4x5 or 8x10 film image.