I found a very easy solution for having NetworkManager installed on Slackware Linux, even without using dirty tricks as I did with Slackware 13.0.
How? Take a look at rworkman’s packages for XFCE 4.8.0 😉
In the extra/packages you will find packages for NetworkManager, both the system and the applet.
I would also install the packages in the packages/*arch*/deps/ folder, since there are some that will surely be used by NetworkManager (all Gconf/Gnome related). I cannot provide the complete dependencies list since I also installed the whole XFCE 4.8.0, but if you find them out, write them as a comment.
This solution works for both x86 and and x86_64 architectures.