Sony Ericsson Xperia X10 :: Can X10 Be On Auto Lock When It's Inside My Pocket?

I usually tap the off button to turn off the screen when I put my phone inside my front pants pocket. I even have the security feature on so when I turn on the phone, I first have to draw a pattern to unlock it. However, I've noticed that sometimes after I've done all this, my X10 seems to be doing stuff on it's own, ie) downloading pic from the net, sending random text messages, randomly calling contacts. I know because sometimes after I "unlock" my phone, I see that the phone is in the middle of doing something. I thought by turning on the security feature, it's supposed to prevent accidental taps on the phone and doing random things while inside the pocket.