Well, all the howtos I could find about this were telling me to download boot.img.gz, zcat it to the usb key's device, then mount it and copy debian's ISO file in it.

It dosen't work [anymore] !

Here's a quick rectification howto on how to create a Debian squeeze installation USB key:

1. download your preferred debian ISO file. I'll be dowloading the current stable netinstall cd (change the version # accordingly):

   wget http://cdimage.debian.org/cdimage/release/current/amd64/iso-cd/debian-7.3.0-amd64-netinst.iso

2. Now, just write that directly to your usb key device (watch out here and make sure to choose the right device since you could be erasing everything from a disk in your computer:

   sudo sh -c 'cat debian-7.3.0-amd64-netinst.iso > /dev/sdb'

3. (Optional, depending on your hardware) I have a Realtek NIC on my Mini-ITX board... so annoyingly, I need to download an additional firmware. But, no sweat! Only need to create a 2nd partition on the key and download the firmware there:

   sudo fdisk /dev/sdb # n -> 2 -> [accept default starting block] -> [accept default to take up the rest of the key] # t -> 2 -> 6 # w partprobe sudo mkfs.vfat /dev/sdb2 sudo mount /dev/sdb2 /mnt $(cd /mnt; sudo wget 'http://ftp.us.debian.org/debian/pool/non-free/f/firmware-nonfree/firmware-realtek_0.35_all.deb') sync sudo umount /mnt

Done! you can now boot that key up and you'll have the installer with the right firmware available.

Update: I've changed the image file for debian wheezy. You can still get squeeze CDs from http://cdimage.debian.org/cdimage/archive/