Just when I get a nice system set up here, lo and behold, Mozilla now has a 64-bit Linux plug-in available. It can be obtained from their FTP site (link for version 0.5). I'll continue to keep the version I have here available, in case this dries up. (UPDATE: It's gone.) But, unless that does, this is probably the last post you'll see here about the 64-bit Lightning plug-in.