Thanks for sharing!
Although I did not notice memory leak for my part, I use xfce everyday
(but I have to say I have not used big x apps since I upgraded to Slackware 12.1 and I power off my computer every night)
[edit]
Slight error in slackbuild script:
./xfce.SlackBuild: line 143: syntax error near unexpected token `)'
./xfce.SlackBuild: line 143: ` ) || exit 1'
In fact it is a "fi" on line 97 that should be put on next line