Quoting Frederik Kriewitz’s reply on debian-user mailing list:
Download the 64 Bit plugin from
http://labs.adobe.com/downloads/flashplayer10.html
extract it and copy and copy libflashplayer.so to /usr/lib/iceweasel/plugins/
Update (11/25/2009):
The new link to 64-bit version is:
http://labs.adobe.com/technologies/flashplayer10/64bit.html