Well how about "the Absolute Maximum operating voltage is exactly 6.0V" as stated by the datasheet. But that will not immediately destroy the device. Anything above the 6V specified in the datasheet qualifies as tempting fate and scraping by in my book. 7V certainly fits that description so..... I assure you that the Arduino was never intended to run on anything that high. I don't know what kind of regulator you were using, but if it was letting 7 or 8V thru then it wasn't working as a "5V regulator", regardless of what you might believe.
EDIT: To try and better answer your question, there is no stated specification whereby the chip is guaranteed to be damaged or destroyed. There is only the Absolute Maximums which are not supposed to immediately kill the device, but are not guaranteed to keep doing so in the long run. Can you now see why I would consider 7V or 8V survival as scraping by or tempting fate? 8V would be exceeding the Absolute Maximum by 33%....
Also, when I say that "I could see" something, I'm generally saying that I might believe that it's true, but that I'm still somewhat skeptical about the point. So, yeah I could believe that it's possible that 7V didn't kill the board, but I wouldn't count on it to apply across the board to all devices manufactured. That's why I said it scraped by. I still would like to see a video of a chip running on 8V.