pretty sure the only way out is the power button (unless you actually have a gateway card)
edit: yup. just double checked. hold the power button for 3-4 seconds and the screens will turn off. music will still play, but it'll stop after about 7-8 seconds. the power led will turn off shortly after.
double edit: this got me the first time; don't freak out if the exploit doesn't fully load the next time you trigger it, just hold down the power button again. after force-quitting the gateway menu, i usually have to trigger the exploit twice before it'll work. the exploit itself is not 100% stable.