  The set of all permutations, ordered by pattern containment, is a poset. We
give a formula for the M\"obius function of intervals $[1,\pi]$ in this poset,
for any permutation $\pi$ with at most one descent. We compute the M\"obius
function as a function of the number and positions of pairs of consecutive
letters in $\pi$ that are consecutive in value. As a result of this we show
that the M\"obius function is unbounded on the poset of all permutations. We
show that the M\"obius function is zero on any interval $[1,\pi]$ where $\pi$
has a triple of consecutive letters whose values are consecutive and monotone.
We also conjecture values of the M\"obius function on some other intervals of
permutations with at most one descent.
