Since Arch's official kernel26 package 22.214.171.124-1, IPv6 is no longer compiled directly into the kernel, but as a module. Many don't require the features, and may benefit from added performance (many programs will query IPv6 addresses first, unaware that you don't have an IPv6 connection) and free memory (250k, that's a mighty big module).
The ipv6 module is loaded at boot. There are many programs which will load the ipv6 module if they detect that it's available. In fact, they're loading net-pf-10, which is an alias to ipv6. Adding the following line to /etc/modprobe.d/modprobe.conf will disable the automatic loading of ipv6, while also allowing you to load it manually, if needed.
# disable autoload of ipv6 alias net-pf-10 off