Here's a fairly efficient Python (2.5) and well-documented implementation of the Rabin-Miller primality test, based on section 33.8 in CLR's Introduction to Algorithms. Due to Python's built-in arbitrary precision arithmetic, this works for numbers of any size.

from random import randint

def _bits_of_n(n):
    """ Return the list of the bits in the binary
        representation of n, from LSB to MSB
    bits = []

    while n:
        bits.append(n % 2)
        n /= 2

    return bits

def _MR_composite_witness(a, n):
    """ Witness functions for the Miller-Rabin
        test. If 'a' can be used to prove that
        'n' is composite, return True. If False
        is returned, there's high (though < 1)
        probability that 'n' is prime.
    rem = 1

    # Computes a^(n-1) mod n, using modular
    # exponentation by repeative squaring.
    for b in reversed(_bits_of_n(n - 1)):
        x = rem
        rem = (rem * rem) % n

        if rem == 1 and x != 1 and x != n - 1:
            return True

        if b == 1:
            rem = (rem * a) % n

    if rem != 1:
        return True
    return False

def isprime_MR(n, trials=6):
    """ Determine whether n is prime using the
        probabilistic Miller-Rabin test. Follows
        the procedure described in section 33.8
        in CLR's Introduction to Algorithms

            The amount of trials of the test.
            A larger amount of trials increases
            the chances of a correct answer.
            6 is safe enough for all practical
    if n < 2:
        return False

    for ntrial in xrange(trials):
        if _MR_composite_witness(randint(1, n - 1), n):
            return False

    return True

The function you should call is isprime_MR.

Although this test is probabilistic, the chances of it erring are extremely low. According to Bruce Schneier in "Applied Cryptography", the chances of error for a 256-bit number with 6 trials are less than one in - this is very low.

Therefore, you should always use this method instead of the naive one (trying do divide by all primes up to ), because it's much faster.