This image was scanned in at 75 dpi - the closest that my scanner lets me get to 72. Whilst this looks fine in the small format of a blog or other fancy internet stuff, when you have it as an A4 page you really see the difference - terribly pixellated.
150 dpi - Again, the same kind of quality once it's shrunk down to a tiny little internet thing. No real need to scan things in at this size, since memory is so cheap! However, in this case it's not really visible. This would probably be OK to print off, but 360 dpi would be nicer.
Ahh, 300 dpi (I'm working on inputting the amount of dpi on my scanner, but no luck so far). In essence, all we should ever be scanning with, if not more. So lovely and clear, all the edges perfect - and you can always make smaller, but you can't enlarge a small crappy image. These kinds of images can be blown up to make posters, used wherever you so choose. Happy days.
Image - Australian Vogue, March 2007.