Scipy LinearOperator dtype unspecified

(This is more of an extended comment than an answer.)

What version of scipy are you using? I'm using scipy 0.13.0. If I don't specify the dtype when I create the LinearOperator, I get the dtype error that you get. But specifying dtype='float64' works for me:

In [1]: import numpy as np

In [2]: from scipy.sparse.linalg import LinearOperator, gmres

In [3]: def mymatvec(v):
   ...:     a = np.array([[4,2,1],[2,2,1],[1,1,1]])
   ...:     return a.dot(v)
   ...: 

In [4]: A = LinearOperator((3,3), mymatvec, dtype='float64')

In [5]: b = np.array([1,2,3])

In [6]: gmres(A, b)
Out[6]: (array([-0.5, -0.5,  4. ]), 0)