My HW50ES has a similar bow at the bottom. I figured it was a problem with my setup, even though everything appeared to be level. I spent a ton of time attempting to correct the issue, but ultimately gave up. The thought never crossed my mind that the problem could actually be with the projector itself, until I stumbled across this thread.
The bowing used to bug me, but I don't really focus on it anymore. At this point I don't want to bother with the hassle of removing the projector from the mount, shipping it, etc... especially since Sony could just claim that it falls within acceptable limits.