Received these from the E-Flight email distro list:

There have been many versions of a servo torque calculator on the web. Chuck Gadd has one on his web site. http://www.csd.net/~cgadd/eflight/calcs_servo.htm

Torque(oz-in) = 8.5E-6 * ( C2 V2 L sin(S1) tan(S1) / tan(S2)]

Where:
C = Control surface chord in cm
L = Control surface length in cm
V = Speed in MPH
S1 = Max control surface deflection in degrees
S2 = Max servo deflection in degrees

And a slightly more complicated one:
http://www.geistware.com/rcmodeling/calculators.htm

The mathematical model: "t = (AMPC2LV2) / (4RT)" where:
t = servo torque
A = sin(S) * tan(S) / tan(s)
      S = control surface angle from neutral
      s = servo arm angle from neutral
M = molecular weight of air (~28.6 g/mol)
P = air pressure (1 atm)
C = average chord length of control surface
L = average length of control surface
V = airspeed
T = air temperature (~290 K)
R = ideal gas constant (82.056 atm cm3 / mol K)