I was doing some low-level hacking on image support and DPI, and found an interesting thing

Images I created in The GIMP and set to 72 DPI there came out to have a stored value of 2835 pixels per meter. On the other hand, a png that had been created in Inkscape a while back had a stored value of 2834 pixels per meter.

One saved today had no chunk for DPI, at least not before the image data.