Old thread, but I thought I'd share my experience this year. I haven't taken perfect stats, but I would guesstimate very conservatively that I have been "randomly" selected for secondary screening about 50+ times out of roughly 100 tries. What then happens is mostly dependent on the airport. At EWR they started the year requiring hand swabs and have since mived to NOS mandatory upon random beep. At AUS, hand swab has been customary but today I received a quick patdown instead.
But main thing to know is that I doubt there is anything random about these secondary screenings.