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Sure, DPI is a good measurement for scanners, because they're needed to convert the document/photo input into digital form, DPI values come with it to make sure they can be printed again when needed. What I mean, is that with digital cameras, even if you know the DPI value (and most cameras I've seen set it to some constant value not depending on the size of the photo), it really wouldn't show anything. What I agree with, is that the photos can be classified by their measurements in pixels. But DPI is only useful for getting the photo to print in the right size without resizing the photo, and most of the photos taken with digital cameras never get printed. (Your examples about digital photo DPI values are all about printing them, except for your example about 75 DPI photos for the web, but most of the mainstream browsers never use the DPI value that comes with the photos, making a DPI value pointless.)

I haven't really seen DPI used with digital cameras, either, but this doesn't mean it's not being used.

Hiya,
IMHO images 'acquired' from any source must be classified by the 'medium of highest resolution'. With any imaging (analogue or digital) this must be paper (digital doesn't quite cut it yet). And paper resolution is measured in DPI - nothing else.

This is why if 'imaging', the camera must have the necessary amount of resolution to satisfy the printing requirement.
'Viewing requirements' are secondary i.e. if the image is to be viewed 'web-wise' it is better to save a lower DPI file for use in that medium, but not to deliberately take an image at the lowest resolution first for that medium (otherwise we would all still be using 1 megapixel cameras).

This brings us back to resolution - on a 6" (15cm) by 4" (10cm) photo (standard photo size in Australia, not sure what it is elsewhere) the dpi of a 5 Megapixel camera (2592 x 1944 pixels) works out at 432 x 486 dpi. This is what I would consider to be 'minimum' acceptable quality as inkjets can produce reasonable to good images at their printing resolutions.
A1 printers such as the HP DesignJet have a DPI rating of 2400 by 1200. Images on these type of machines MUST have as much information as possible.
Example:
Images acquired from LANDSAT (used for geographical surveys) have a typical resolution of 3600 dpi with each single image requiring 1.3Gbytes of storage (Black & White).
The image at 3600 dpi has a pixel size of 57 meters.

So I stand by my point that eventually as higher resolutions are achieved, dpi from digital cameras will be more important that actual pixel resolution.
As for dpi only being important for printing, I'm sure that any desktop publisher would know the DPI of their monitor and how the image resolution relates to internet publishing as well as the standard 'print media' (a 1600x1200 image on a 19" monitor as a horizontal DPI of about 84). Imagine trying to view a LANDSAT image on a monitor (LCD, CRT, whatever). The best way to view such digital images is to perform the processing, then print.
So by extension I'll make the prediction that as monitor resolution increases - so does the importance of dpi. Web browsers may just start using that dpi setting altering the image displayed depending on the capabilities of the viewing device. In fact some browsers already do, such as those on PocketPC's and Mobile Phone Web access.

Of course having enough memory and storage space to 'play' with the images to make sense of them is another point entirely. Where's that 1Tbyte of RAM?

Hey, only thing I actually do not agree, and tried to explain before (though it seems, quite miserably), is that as a collection of bytes, a digital photo has no DPI, it's assigned to it when you want to print it (or look at that photo on screen, for that matter), therefore, one can't classify digital cameras with a DPI value. I agree with everything else you've said so far about the topic, but seeing as you replied to me as if I had said DPI values were pointless (which I did not ), I thought it was better to rephrase my original point.

Humble apologies - I've been eating so much turkey recently, it must have gotten into my brain (the old 'You are what you eat.')

@Eero:
My DPI is getting confused with PPI (points per inch). I've never really considered acquiring an image without considering what I'm going to 'potentially' print it on.

To paraphrase and hopefully confirm your point:
"If you're just taking an image, DPI is useless unless you're going to print it because the PPI value (camera resolution) has no correlation to the DPI value unless the input and output resolutions are exactly the same."

OK, I get this. I'm still old-school 'can't believe the world will never have paper', but I can see how in a purely digital (yechhh) world, some people may never print. Considering the massive power outages experienced in my part of the world, I'd prefer my info in paper though as I can't spin the CD fast enough to see the picture properly. ;D