The DPI can also be set using the DisplaySize parameter in XF86Config-4. Although this seems to be semantically equivalent to the -dpi option (none of my computers auto-detect it, so I can't tell if auto-detection overrides DisplaySize, but it seems unlikely), it would be 1) easier for users to find and change, and 2) easier to handle with dexconf. A simple 'I have a 14"/15"/17"/21"/custom/autodetect monitor' question would make this issue go away.
This suggestion comes from the perspective of one who always enters DisplaySize manually, and removes the -dpi from xserverrc. Monitors differ enough in their resolutions these days that it's really worth getting it right. Also, has anyone asked the X developers for a "soft" version of -dpi or DisplaySize? It seems like a trivial feature. Andrew
