(Presumably) in anticipation of the move from X11R6.9 to X11R7.1, fluxbox-devel no longer installs the fluxbox binary in /usr/X11R6/bin, installing it more apropriately in /usr/local/bin. As a result, if you portupgraded fluxbox-devel and found that you could no longer startx, here's what you need to do:
wget http://people.freebsd.org/~mezz/script/fluxbox_prefix.sh
sh fluxbox_prefix.sh
NOTE: If you don't have wget installed, you can also download the fluxbox_prefix.sh script with this command:
fetch http://people.freebsd.org/~mezz/script/fluxbox_prefix.sh
After which you can run it just as before:
sh fluxbox_prefix.sh
You should then be able to startx normally, booting into your favorite window manager.
Assuming you are able to boot into X/fluxbox without any problems, in a new terminal window, type:
rm -r .fluxbox-bak
Which will remove a backup directory created by the fluxbox_prefix.sh script.
Please NOTE that this information is available in /usr/ports/UPDATING, which is the first place you should check when you have trouble with an upgrade.