Could it be that one of your extensions has gone bad? Look for your Extensions folder inside ~/Library/Safari/ - remove the folder by dragging it to the desktop and start over. It could be that it's an extension you have installed is causing this or it might not be.
Or maybe you don't have any extensions installed at all yet!? In that case try removing your com.apple.Safari.plist from ~/Library/Preferences/ - this will reset Safari's configuration file and you will probably see the Welcome to Safari animation once again.