After upgrading to Safari 5.1, new windows in Safari won't show up, although new ones are created according to the Windows menu, dialog boxes cannot be closed using the dialog controls, all windows except the initial window are unresponsive. After disabling all extensions and plugins, resetting Safari, deleting preferences and the Safari Library folder to no avail, I remembered Web Snapper uses SpiceRack, so I disabled the Web Snapper plugin in system preferences and voila, Safari is usable again.
Looking forward to a Safari 5.1 ready version of Web Snapper.