Being that it is a userland jailbreak, it is believed that Apple will provide an update to patch the exploit immediately after the release of Spirit. Once Apple releases a new firmware you can no longer get the SHSH file for an older firmware, thus you can no longer install the old firmware if something goes wrong.
Is that true, though? Doesn't the umbrella thing work anymore or something? I mean I guess I don't see how a new firmware being released has any effect on my getting the ECID SSH file from my iPad running a release firmware.