Just had an additional thought on how its unlocking without me noticing.

I use a case whch i slide the n900 into like a leather sock. The end i always leave facing out is the end with the charger usb connection so i can charge it in the case.

One way to unlock the phone is to press the power button then slide the bar across.

As i pick the phone up in the case i may be pressing the power button and as i slide it out of the case the case pressing against the scree maybe unlocking it. Then i may not be using the phone and it may go back into the case or pocket or whereever and i dont realise its unlocked itself! Then any pressure and movements are moving the widgets around willy nilly!