Depends on the camera and the settings. Basically, what you get with a digital camera is a set amount of pixels - I generally take my photos at a 1024x768 resolution, medium quality (good enough for me) and the maximum for my camera is 2048x1536, I think - for your average photo needs, this is more than good enough.
But this also means that once you try to enlarge it by a lot, pixelisation kicks in. You can enlarge it a bit and not notice the pixelisation with a naked eye, unless you have a really good eye for that sort of thing, but not by too much.
With a regular photo, a lot depends on the scanner - the better the scanner (the better the maximum resolution it gives), the better the scanned image will be. But of course, there's only so much you can do by scanning in for example a 4''x6'' photo. Of course, if you have the negative for that photo and use a good professional scanner, you'd probably get better results with enlarging than with an amateur digital camera.
Or something like that. /forums/images/icons/grin.gif I'm no expert, I'm just talking from my personal experience, having both a fairly decent amateur user digital camera and a fairly decent amateur home user scanner.