It's permanent in that if you flash a different hboot, you'll still have S-OFF. The *normal* way to get S-OFF (more appropriately, the way it used to work) was to flash the eng bootloader. When you did that, changing hboot would unroot the phone. Now, if you flash a different hboot, you'll still have S-OFF. I'm not saying you should ever consider flashing a different hboot, as there are other issues that go along with that. It's just the way it works. Beyond all that, though, once you flash the S-ON tool, it will undo whatever unrevoked-forever does to get S-OFF. It's lower than hboot, so you must flash it. Once that is done, and you flash a RUU, it will undo everything that was done to root. From a strictly nand, recovery, and ROM point of view, it looks like it never had root. It's up to you to remove the rest of the traces (if you're that paranoid). Things like flashable zips, nand backups, etc. left behind are kind of a dead giveaway. That being said, who's going to actually look on your sdcard? It's up to you how far you want to go beyond the normal unrooting steps. But, that being said, it's *not* permanent. It just takes deliberate action on your part to unroot it.
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