Ack... I'm an idiot:
Skip> def safe_primes(last=1):
Skip> last = long((last-1)/2) # assuming future float division...
should be last = (last-1)//2 # doh!
Skip> while True:
Skip> last = gmpy.next_prime(last)
Skip> other = last * 2 + 1
Skip> if gmpy.is_prime(other):
Skip> yield other
Skip