Adobe has discontinued 64-bit Flash 10.1 development for Linux stating that significant architectural changes were being made to the plugin that would add improved security.
"We are fully committed to bringing native 64-bit Flash Player for the desktop by providing native support for Windows, Macintosh, and Linux 64-bit platforms in an upcoming major release of Flash Player," Adobe wrote on their Adobe Labs site.
The 64-bit plugin was first made available for testing two years ago, but a finished, official 64-bit plugin never materialized. It's uncertain what caused this setback at Adobe, but speculation points to Adobe's focus on Flash 10.1.
Adobe gave no timetable for when it would begin development (or redevelopment) on a 64-bit plugin.