How do I mount a USB in plain XMonad?

I'd recommend one of the udisks-based solutions. uam seems to do the job without too much configuration. udisks-glue is also decent for scripting.

• uam:

This is just simple tool for mounting pendrives and alikes when inserted -- something like ivman or functionality integrated in KDE/GNOME.

• udiskie:

udiskie is a simple daemon that uses udisks to automatically mount removable storage devices.

The way that I have solved that problem is to use nautilus running the the background to continue to manage that for me. You can do that like so:

nautilus --no-desktop -n &

If you happen to start XMonad using a custom .xsession file then that would be the perfect place to put that line. Make sure you put it before the 'exec xmonad' command.