Let's get one thing clear: WikiLeaks is not entirely about freedom of expression. It is also about publishing stolen information. It does, however, raise an interesting question about who should have what legal entitlements to do what with intercepted or stolen diplomatic communications. This point is made quite forcefully by Janet Daley in The Telegraph.
In its self-contradictory maintenance of its own untraceable operations, it [WikiLeaks] effectively declares itself to be the only agency in the world that is entitled to secrecy. Its insistence that it is somehow a voice of open and transparent “freedom of expression” is simply absurd: there is no issue here of any individual or group openly expressing an opinion that would otherwise have been suppressed. The only opinion that is implicitly conveyed by WikiLeaks’ exposures is the boringly prosaic anti-Americanism of the average Guardian comment writer.
All that WikiLeaks has done, as its name suggests, is to publish stolen documents that were purloined by a malcontent within the US defence network. As it happens, the leaked material has been almost entirely unsurprising, apart from one rather spectacular own goal in WikiLeaks terms: it turns out that a number of Gulf states have been urging the US to strike at Iran before it succeeds in producing nuclear weapons, and that the US has been resisting this pressure. This tends to undermine both the image of America as trigger-happy warmonger and the idea that the entire Muslim world is united in hatred and distrust of the Great Satan.
There are, of course, at least two parties involved in the WikiLeaks: those who "stole" the documents and provided them to WikiLeaks; and WikiLeaks itself. WikiLeaks is probably more like a publisher than anything else and will quite possibly be protected under freedom of the press. At the same time, I'm not sure that freedom of the press protects disclosure of "official secrets"; my recollection from The Pentagon Papers escapade of 1971 is that it does in at least some instances (for example, see this); but even if it doesn't, if the only relief is injunctive, it's too late now -- the cat's already out of the bag.
Update: George Friedman of Stratfor has pretty similar views.