Installing Fedora from USB key

The second command is the correct. Try this:

sudo umount /dev/sdb*
sudo dd if=/dev/zero of=/dev/sdb
sudo sync
sudo dd if=Fedora-15-x86_64-netinst.iso of=/dev/sdb
sudo sync

If you have the same errors, try the GNOME or KDE livecd in your USB stick. If all fail, the problem is your hardware.

There is loads of useful information on http://www.pendrivelinux.com/category/usb-installs-from-linux/

Ubuntu comes with a utility called Startup disk creator which you could use from a live ubuntu CD.

I always use unetbootin though as you can play about with the settings a bit more and make live USB with persistent storage and carry your desktop in your pocket.