When installing arch using the current -rc isos (with [core]), I chose not to use DHCP. Setting up ip and netmask worked, but neither the default gw nor the nameserver were set. The installer tells you that he failed to set the route. After manually executing route add default gw foo and creating /etc/resolv.conf the network works just fine.

Additionally, the installer asks if you want to keep the downloaded packages. Answering 'no' does not seem to have any effect, they are still in /var/cache after the reboot.