ImageKind, who have a reputation for high quality prints, say that improvements in printing technology, inks, and paper, has meant that printing at 137dpi is perfectly good enough. They seem to think that a 6mp camera is capable of quality prints up to 24in x 16 in.

http://www.imagekind.com/printing.aspx

I see that ImageKind refer to inkjet technology for 137dpi. Glossy Photo magazines are printed using halftone technology which are typically printed at 150dpi or greater. Halftone technology required the image to be at least double the dpi of the halftone frequency. Hence the requirement for 300dpi images.

Other than double spread, do glossy photo magazines normally print at anything approaching 24 x 16?

If you take a Glossy magazine printing a 15" x 11" photo @ 300dpi you would need a 14 Mp camera to Produce a 42Mbyte uncompressed file.

If you take newsprint which typically uses a 120 dpi or less halftone.

A spread in a tabloid 22" x 14" will require a 51Mbyte uncompressed file.

A spread in the Berliner Format (Broadsheet replacement) 18" x 24" will require a 71Mbyte uncompressed file.

I can recall when visiting a customer ( an advertising agency ) the images for the A0 bus shelter adverts were about 350Mbytes.

And yet Virgin Mobile managed to take half of this photo:
http://flickr.com/photos/chewywong/467623403/

and turn it into this:
http://flickr.com/photos/sesh00/515961023/

As the information in the image is low resolution (not very detailed) it certainly works in this case. Not so sure it would work so well with a highly detailed image.

Food for thought as to what is possible with 'small images' nowadays though. That 1000 pixel wide image can be enlarged quite a bit:
http://www.dpreview.com/news/0708/07...geresizing.asp

Meanwhile newspapers have been known to steal 500 pix images for their pages:
http://www.flickr.com/photos/duncan/379512083/

not high quality glossy I know but still ...

If you need bigger files, have a look at this upsampling technique on Digital Outback. I've used it with good results to make a 90 cm wide image from a file only 3100 pixels wide - if I had made a straight print that would have only been ~85ppi. It also useful for upsampling to send images to labs that demand 300ppi images.

There is even a photoshop plugin to do the work for you :D