Download the beta. Pop in your card
sudo dd bs=1m if=wheezy.img of=/dev/disk1
where wheezy.img is the unzipped image file – I renamed because I am lazy and there is less typing involved 🙂
/dev/disk1 is the name of the SD card without the s1.
It does take AGES to write.
You can also update the firmware by grabbing it from here https://github.com/raspberrypi/firmware and following instructions here.