Hi,
I was thinking about adding <div align="left"> to the header of the guestbook (and not closing the tag), but in Internet Explorer 7 Beta I was not able to get this to work. I'll see if this can be added as an option to in the Control Panel but this feature will not be ready for some time.
If anyone else has a smart solution to left/right align the guestbook feel free to post a message here :)