Here is one of the responds I got on my website/forums.

"Having worked in newspaper layout....I disagree. You really do need a higher dpi when printing something for it to look decent, even if it is a digital photo. I think it's gotta be the camera settings and/or software. Check your camera manual again. My Nikon is only 6 MP now that I think of it but I get 300 dpi pics because I have the pic quality set to the highest."

I have a Canon Rebel XT and evn at 8MP quality set to Medium I still get 72DPI Photos just at higher screen resolutions like 2500x1450 etc..

I just to think I had this DPI and screen resolution thing figured out, but I havent.

There does not seem to be a set rule of thumb.