As it turned out, my report of the death of Cosmos-2422 early-warning satellite was an exaggeration - the satellite is alive and well. It did behave a little bit unusual, not performing station-keeping maneuvers between August 2009 and May 2010, but it has been performing well since then.
This means that the early-warning constellation includes one more satellite than I thought it did when I was reporting on its status in April 2010 and then in September 2010. There are four operational satellites on highly-elliptical orbits now. No geostationary satellites are operating, though.
The book
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Hello, Pavel
You write in some other place, that three early warning satellites cover US 18 hour a day. Does it mean, that together with fourth alive and well there is round the clock coverage?
It's a bit complicated. Four HEO satellites should be able to provide 24 hour coverage of the U.S. territory. However, with just four of them, most of the time it's one satellite that is looking at the launches, so the probability of detection is not very high because of reflections and things like that. Normally, you would want at least two (better three) satellites to see every point within the coverage area. But one would do the job most of the time.