I don’t know if this is Arch Linux specific or not. After a recent kernel upgrade, 2.6.28, I was not able to connect to hidden wireless networks. This could either be fixed by unhiding the SSID, or by the steps below.

Install the “crda” package:
# pacman -S crda

Uncomment your region in the “wireless-regdom” file:
# vi /etc/conf.d/wireless-regdom

Add “wireless-regdom” to the daemons line of rc.conf:
# vi /etc/rc.conf

DAEMONS=(syslog-ng hal dhcdbd !network wireless-regdom networkmanager rpcbind nfs-common @alsa @openntpd @netfs @crond gdm)