I finally bought a USB WiFi adapter for my Raspberry Pi. It was so cheap and easy to set up that I definitely recommend it if you're thinking about getting one. I got the "Edimax EW-7811Un 150M 11n Wi-Fi USB Adapter" (which you can get on Amazon here).
I then edited /etc/network/interfaces to look like this:
iface lo inet loopback
iface eth0 inet dhcp
iface wlan0 inet dhcp