Hey folks, my S2 has done this as well. It seems to do it when i tap an app and it doesnt matter which one i dont think. Last time was when i went to open Twitter and the screen went black and i got the samsung start up screen followed by the 02 welcome message.
I have tried taking battery out and putting back in to ensure its in firm, ive rebooted it myself every night in case its the RAM getting too clogged even though i clear the RAM regularly.
I have an appointment with 02 tomorrow to get this sorted so will report back when i know whats what. Although them replacing it would be good, it would also mean i have to pay another £12 screen protector.