A few moments before President Reagan stepped in front of the television cameras, accompanied by Treasury Secretary Donald Regan and White House Chief of Staff James Baker, to announce that the two men would be swapping jobs, each of the three big networks -- ABC, NBC and CBS -- made an announcement of its own. Each told its viewers that in the next few moments Mr. Reagan would be on the air to issue a statement; each said the subject of the statement was unknown; but each surmised or suggested that the subject would be the Geneva arms talks.

So much for the omniscience of the press. So much for Mr. Reagan's complaint that you can't do a darned thing in this town without somebody or other's leaking it, the rats. The fact is that where it wants to, this administration has really been able to mind its own business and keep outsiders from minding it as well.

Before the TVs all start screaming, we hasten to add that with certain exceptions (Lou Cannon, to take a case) we would include the good old writing press in this assessment: by and large an elephantine Washington press corps has been effectively closed out of much of the internal administration deliberation it desperately wants to cover. Mr. Reagan is good at this. If the account put out about how the Regan-Baker switchover decision was reached is accurate, you will see one reason why: there is no strategem available for predicting something as unconventional as that.

Another reason for the newsmakers' success may be that these people in the administration are getting better at keeping their mouths shut. A final reason may have something to do with the press corps' size: government figures are much likelier to loosen up with and confide in small groups of reporters than they are when there is a cast of thousands. The scene in Geneva, where hundreds of journalists reportedly milled about in hotel lobbies and other such settings, frozen out of the action, is a case in point. We have swelled in numbers, but not therefore in productivity. The media, alas, milling not just by the hundreds but the thousands through political conventions and presidential events and financial summits and the rest have taken on the aspect of an operatic chorus moving noisily across the stage, swinging axes or brandishing swords and shields or whatever the opera calls for, but often ending up with a relatively minimal part for all their great number. Mr. Reagan, it seems, still gets to sing the arias.