If you're having a weird problem with evince showing gray boxes instead of text in some pdfs, the problem could well be a bug in cairo.
After some help from "danigm" on the #evince channel on irc.gnome.org, I found that the bug disappeared on upgrading libcairo to 1.9.8 (which is currently in debian experimental).
Anyway, hopefully the next person who searches google for grey boxes in evince or indeed gray boxes in evince should find the solution!
