The plural of Octopus is Octopi. [Update: 7 August 2010] Octopi is one of several options. Here is a clip explaining it in etymological terms:
In Math, Pi is a mathematical constant whose value is the ratio of any circle‘s circumference to its diameter in Euclidean space; this is the same value as the ratio of a circle’s area to the square of its radius. It is approximately equal to 3.141593 in the usual decimal notation.
This is the symbol for Pi:
If a plural form of Pi existed, would it be this?: