Stanley likes to keep a list of combinatorial results for which there is no known combinatorial proof. For example, until recently I believe the explicit enumeration of the de Brujin sequences fell into this category (but now see arXiv:0910.3442v1). Many unimodality results also fall into this category. Do you know of any other results of this kind, especially results that look frustratingly like they ought to have simple combinatorial proofs?

For the purposes of this question, "combinatorial result" should be interpreted as meaning some kind of exact enumeration, and "combinatorial proof" should be interpreted as meaning, more or less, "bijective proof." (So for example I am not interested in bounds on Ramsey numbers.)