Hi,
I had a discussion with several people at the SVG Working group meeting in Leipzig this past week about user units vs. "real" units. I proposed that it be possible to set a ratio for the initial user unit to inch (or cm). This was not well received. My basic conclusion from these discussions is that the use of any real unit (inch, pt, etc.) inside an SVG file (other than on the root 'width' and 'height') is pointless. The correct way to proceed is to set the 'width' and 'height' of the SVG to the size of the drawing and then the 'viewBox' to the same values without units, e.g.:
<svg width="20in" height="15in" viewBox( 0 0 20 15 ) ... >
Once you have done this, one user unit is one inch. If you actually set a length to an inch inside the file (i.e. width="1in"), it will be interpreted as 90 inches since we use 90 user units per inch.
OK, so be it. The gods have spoken.
However, we should at some point change 90 user units per inch to the CSS 2.1 value of 96 which has now been fixed in stone. (Illustrator has the same problem, having used a value of 84.)
Tav