Description:
/usr/share/terminfo/r/rxvt-256color should have max_colors set to 256, rather than the current 88 (as of ncurses-5.7-2). rxvt-unicode-256color from AUR sets the TERM variable to "rxvt-256color", and with the current configuration requires a line to be added to users' ~/.screenrc files to at least get 256 colors working within screen, as well as other configurations (i.e. setting t_Co in ~/.vimrc, etc).

Note that, the terminfo db being incorrect doesn't prevent 256 color support, but ncurses doesn't know about the true number of supported colors. That's why things such as the xterm 256 color test script worked, but `tput colors' outputted something less than 256 (8, 16, 88...)

To check:
$ infocmp -L rxvt-256color | grep "max_colors"
lines_of_memory#0, max_colors#88, max_pairs#256,

To fix (manually):
$ infocmp -L rxvt-256color > /tmp/rxvt-256color.terminfo
$ [editor] /tmp/rxvt-unicode.tmp
Find max_colors, set it to 256
Find max_pairs, set it to 32767
$ tic -o ~/.terminfo /tmp/rxvt-256color.terminfo
$ export TERMINFO="~/.terminfo"

Steps retrieved from http://lists.schmorp.de/pipermail/rxvt-unicode/2008q2/000626.html