I just ran
df -h a minute ago and noticed a filesystem has been added that I'm not familiar with. Does anyone know why
/run exists? Is this something that's
been added by the kernel? By Arch Linux?
run 10M 236K 9.8M 3% /run
Apparently, many tools (among them udev) will soon require a /run/ directory that is mounted early (as tmpfs). Arch developers introduced /run last month to prepare for this.
The udev runtime data moved from /dev/.udev/ to /run/udev/. The /run mountpoint is supposed to be a tmpfs mounted during early boot, available and writable to for all tools at any time during bootup, it replaces /var/run/, which should become a symlink some day. 
There is more detail here: http://www.h-online.com/open/news/item/Linux-distributions-to-include-run-directory-1219006.html
 From thread on the Arch Projects ML
/run directory is the companion directory to
/var/run. Like for example
/bin is the companion of
That means that daemons like
udev, which are started very early in the boot process - and perhaps before
/var/run is available (i.e. mounted) - have with
/run a standardized file system location available where they can store runtime information.
/bin contains important programs, which may be needed in the boot process before
/usr is available (in case it is on its own filesystem).
/run idea is a relatively new idea/standard, one supporter is the developer of systemd.