What I hate is that many well regarded jailbreak sites do not mention this issue. iClarified mentions nothing about it in many of their tutorials I just read, making you think you can jb and easily put a fresh install of, say 4.2.1 on your ipod Touch, (current Itunes only 'signs' installs the newer 4.3.1 on). These sites don't mention, you need to have saved the 4.2.1 blobs from your device before 4.3 came out to be able to reload 4.2.1 again and jb it on your Touch. not one word about it.
It;s complicated, for newbies, but they don't tell you this. It's very misleading.