I haven't installed any screenshot applications, in fact my phone was recently wiped when upgrading to gingerbread. I have Gingerbread 2.3.4 and I use the GoLauncher homescreen. Samsung Mesmerize from US Cellular. I know they install bloatware but I've used this phone for months and it only did this after the upgrade to gingerbread (that I know of).
On occasion when I hit the power button the screen turns white and the phone vibrates, and I get a toast message saying "screen captured saved as image file" or something like that. Half the time the power button does that, half the time it turns off the screen normally. Right this second I couldn't replicate, it's very random and frustrating. What could cause this? I see no unusual running apps unless GoLauncher is doing this, and I see no setting related to this in their app.