Looking through the comments, it seems that usb-creator does not install a new MBR.
I can confirm this for Intrepid. Steps to reproduce:
1. Manually install grub on the USB drive
2. Verify that grub boots correctly
3. Run usb-creator.
4. Try to boot from the USB drive. It only gets up to "GRUB". But if the MBR had been installed, it shouldn't even get there but directly start syslinux.
Looking through the comments, it seems that usb-creator does not install a new MBR.
I can confirm this for Intrepid. Steps to reproduce:
1. Manually install grub on the USB drive
2. Verify that grub boots correctly
3. Run usb-creator.
4. Try to boot from the USB drive. It only gets up to "GRUB". But if the MBR had been installed, it shouldn't even get there but directly start syslinux.