The Greek letter tau is already used to refer to the period of an oscillation, the time constant of a decay interval (these are intimately related), plus plenty of other stuff. There really aren't any Greek letters left that aren't used for a million things already. tau-as-time-constant is the standard use for the thing, and the confusion with torque and natural temperature is bad enough as it is.<p>Yes, pi shows up as the prime counting function, but there it's a <i>function</i>, which clears up the otherwise ambiguous notation. Furthermore these abuses of notation are generally considered a bad thing, something we try to avoid.<p>As for the intuitiveness of such deep results as Stirling's formula and the even values of the Riemann zeta: this is to concern oneself with the upholstery on the Space Shuttle.<p>If you want a new pi symbol, might I suggest the variant pi described here: <a href="http://en.wikipedia.org/wiki/Pi_%28letter%29" rel="nofollow">http://en.wikipedia.org/wiki/Pi_%28letter%29</a> -- though I'm afraid this is all a waste of time and energy.