No, really, pi is wrong: The Tau Manifesto (Tau Day, 2010)
Perfect; to my lisp startup file was just added:<p><pre><code>  (defconstant tau #.(* 2 pi))
</code></pre>
Regarding adoption, I think it's worth pointing out that these sorts of conventions can and do shift within a generation.  I recall quite clearly while growing up the convention for indicating years < and >= the year one [1] was B.C. and A.D.  It seems like over the past few years we've definitively decided on B.C.E. and C.E. instead [2].<p>All it really takes to catch on is adoption by a handful of elementary school textbooks -- and those publishers have adopter much crazier things in the past.  As the manifesto points out, this is distinctly pedagogically useful, so it doesn't seem beyond the realm of possibility.<p>--<p>[1] Correction: Julian/Gregorian years begin with 1 A.D. (I still prefer the AD/BC convention, as the two identifiers have an equal number of characters.)<p>[2] I'll admit that my perception of prevalence here may be strongly influenced by the fact that Wikipedia has adopted this convention.