Why oh why. Sudden oil pressure drop.

I've had a brand new Wix filter on a 2.7 cause this sort of problem (low oil pressure at idle). New Wix fixed it.
I had a bad cam bearing in a 273 cause low oil pressure at idle (missing material from oil hole to air).
Also, assuming hydraulic lifters? Any noise, miss, or other change in the way the engine was running? If it was still running good with no noises, it probably still had oil pressure.