It goes:

How I want a drink, alcoholic, of course, after the heavy lectures involving quantum mechanics!

If you count the letters in each word, you get: 3.14159265358979

This lovely bit of wordplay is attributed to Sir James Jeans (Gardner 1966, p. 92; Castellanos 1988, p. 152; Eves 1990, p. 122; Davis 1993, p. 9; Blatner 1997, p. 112).

Via mathworld.wolfram.com.