I reinstalled old and new wacom driver versions, restarted several times, got back to the latest wacom drivers, and this is the result:
Neither my Graphire nor my Bamboo have pressure in MyPaint now, however the Intuos still works. In SAI, it depends... sometimes the Bamboo has pressure, sometimes it doesn't. Same for the Graphire. However, the Intuos always works. In TwistedBrush, pressure always works for all three devices. Crazy world...
Update: now Bamboo pressure suddenly works in MyPaint. I think it's always working when I have plugged in the Intuos at the same time, when I remove the Intuos USB and restart MyPaint, the Bamboo pressure doesn't work any more. Maybe I have just discovered Wacom's secret plan to sell more Intuos? ;-)