from twisted.internet import reactor, threads

def doLongCalculation():
    # .... do long calculation here ...
    return 3

def printResult(x):
    print x

# run method in thread and get result as defer.Deferred
d = threads.deferToThread(doLongCalculation)
d.addCallback(printResult)
reactor.run()

Но, по последним наблюдениям, callback вызовется не раньше, чем через секунду.