Actually they are already mounted!!!
Here, on a installation of PC-BSD alongside from XP Pro, in LXDE, if I open PCManFM, I can see a partition called "XP" but clicking on it or right-clicking and trying "mount" or "unmount" does nothing...
But if I open a terminal and type "mount", I can see this:
/dev/label/rootfs0 on / (ufs, local, noatime, journaled soft-updates)
devfs on /dev (devfs, local, multilabel)
/dev/label/var0 on /var (ufs, local, noatime, journaled soft-updates)
/dev/label/usr0 on /usr (ufs, local, noatime, journaled soft-updates)
procfs on /proc (procfs, local)
linprocfs on /compat/linux/proc (linprocfs, local)
/dev/fuse0 on /media/XP (fusefs, local, nosuid, synchronous)
So, pointing PCManFM to /media/XP and, here it is
I was also surprised, but it has to do (if I'm right) with the way PC-(Free)BSD handles the mounting of devices 'till hal is no more (more tech stuff about it in the PC-BSD docs on Xfce / the amount script).
That's true for internal ntfs partitions, but if you plug an external ntfs HDD, it's also appears in PCManFM, but you also need then to point manually to /media/[you_device_name] to get to the files.
But this time, you have a nice icon on the desktop to remove it, thanks amount...
Hope this helps...