I just developed a similar slicing problem, and have figured out why. I was doing almost exactly what kiwi-hacker was describing. I just got a new set of offset clubs and was paying close attention to getting my hands through the ball - assuming that the clubface would square up because of the offset. By looking at the marks left by the driving range mat on the bottom of the clubs, I determined that my hands were too far ahead of the clubhead, which left the clubface open.

But, standing too close can also cause a slice. A few years ago when I took a lesson, the pro had me move away from the ball, which cured my slice. Now when I address the ball, I just make sure there is a good 5" between me and the end of the club.