Ok, so let's say I have a 4x6 print made at Ritz from a roll of film. Then I want to scan that print on my flatbed scanner which goes up to 600dpi. Is there 600dpi worth of data in that 4x6 print or are you just telling the scanner to make stuff up when you choose a resolution that high? Would you be able to get more data from the film negative? I think Nikon has a film scanner that claims to do a whopping 4,000 dpi.
If you do a digital print on a home inkjet printer I wonder if that would tend to have any more/less data in it than a film lab print.
if you're doing scanning you might want to check out
http://www.hamrick.com/ (it gives me better scans than the canon photoshop plug-in that came with my scanner).