Creating a Windows 8 bootable usb on Mac 10.6.8 without BootCamp?

None of the answers got it right.
This is how you do it — without BootCamp:

• Get/download a Windows ISO image file.
• Your USB flash drive should have at least 8GB.
• Plug it in and format/erase it using Disk Utility (under Applications/Utilities)
  • CAUTION: Make sure you select the correct drive! Then click erase.
  • Select MS-DOS (FAT) for the Format.
    (If your drive is above 32GB, you might need to select ExFAT instead.)
  • Select Master Boot Record for the Scheme.
    (This is the one used for Windows partitions.)
  • Click erase.
• Open Terminal and run command: diskutil list
  From the output, find out the name of the USB drive. It should be something like /dev/diskX. X is the number of the drive. For me, it's /dev/disk2. It might be different on your Mac.
• Then type command: diskutil unmountDisk /dev/disk2
  It should print something like this: Unmount of all volumes on disk2 was successful.
• Then type: sudo dd if=/path/to/win-iso-file of=/dev/disk2 bs=1m
  (This will ask for your Mac password. Enter it and hit return.)
  • /path/to/win-iso-file is the full path of the ISO image. Just drag/drop it on terminal after if= and it will print the full path.
• The process will start but there will be no output for a while. It might take 10 to 45 minutes. Be patient! (You can press Ctrl+T to see the bytes transferred so far.)
• Once it's complete, it will output records in/out and bytes transferred.
• Type: diskutil eject /dev/disk2
• Now, re-plug the USB drive and check its contents. It should have files and folders but especially a setup.exe and an autorun.inf file.
• Done.

Have you tried using the cross-platform application called "unetbootin"?

Correct me if I'm wrong, but I've been using this method for the past year and a half, and it works quite well. Although, it's a little tedious and involves the CLI/Terminal. You'll have to install the MBR bin files manually with either SysLinux or the ones that are included within the unetbootin.app package contents.

Here is a link to the full instructions from the very popular blog post.

UPDATE: Graham try using this answer: Creating A Bootable USB Of Windows 8.1 On OS X?