Since 0.5 up to 0.10.2, every published Tails i386 ISO image has been a hybrid one:

• it can be burnt to a CD;
• it also contains a full disk image (including partition table) and can thus be copied (using dd) to a USB stick; the USB stick's content is lost in the operation.

Successful tests:

• boot the resulting test.iso as a normal optical drive in VirtualBox
• boot the resulting test.iso as a KVM hard disk
• convert the resulting test.iso, posing as a HDD image, into the native VirtualBox disk format, using VBoxManage convertfromraw -format VDI test.iso test.vdi, then boot the resulting test.vdi disk image in VirtualBox
• dd the hybrid test.iso to a USB stick and boot it on a USB-boot-enabled computer.

Notes:

• the partition number may need to be 4, in order to be compatible with the BIOSes supporting only USB-Zip; reference
• the partition type may need to be 1c, i.e. hidden FAT32 LBA; reference
• boot failures were experienced when testing without the -entry 4 -type 1c options.

Conclusion: this is a nice way to get bootable USB images for almost free.