htc desire z - My sense home screen has stopped auto rotating, how do I fix it?

I have a desire z (aka tmobile g2). When I fist got it a week ago the home screen used to rotate when the phone was tilted. It has since stopped doing that.

Other things still rotate when the phone is rotated. And the home screen rotates when I extend the keyboard. And menu -> settings -> display -> auto rotate screen is checked.