The tuple unpacking is pythonic, but some other parts of your snippet aren't. Here's a slight modification using a generator to avoid (and separate) the string concatenation:
def fib(k=100):
""" Generate fibonacci numbers up to k. """
a, b = 1, 2
while a <= k:
yield a
a, b = b, a + b
print(' '.join(map(str, fib())))
# prints: '1 2 3 5 8 13 21 34 55 89'