Update 9-16-2010: Adobe has released another preview release for 64-bit flash, version 10.2. For most cases I would recommend 64-bit systems try that instead of using this outdated and annoying hack.
This is a copy pasta from my email to the debian-user mailing list.
Because Adobe left us high and dry (and vulnerable) by dropping beta support for 64-bit flash, I cobbled together a solution that seemed simpler to me (at first glance) than the official Debian solution of running chrooted 32-bit browsers with 32-bit flash (howto). Reason being, I need to be able to easily roll out flash 10.1 to multiple systems, and any changes should be transparent to my users. schroot looks like it also fits the bill, but I have not tried it yet. schroot is probably a better solution in the longrun since you can use apt to upgrade all the dependencies without repackaging flash. Hopefully Adobe adds 64-bit linux support soon so that doesn't become an issue.
I simply copied all 32-bit libraries required by flash that are not provided by ia32-libs or ia32-libs-gtk (or other multi-arch support packages) into /emul/ia32-linux/libs. Adobe Flash apparently also requires the curl binary, so I copied that as well in /emul/ia32-linux/usr/bin. I don't know how kosher what I did was, and I was actually surprised that apparently /emul/ia32-linux/usr/bin is in the 32-bit executable path (putting curl in bin/ stopped Flash from complaining that it could not load the cURL library).
Then I used nspluginwrapper to wrap the 32-bit flash plugin binary in a 64-bit wrapper.
If you'd like to try out my solution, it works on my systems in iceweasel, google-chrome, and opera. Here is a link: link removed because it is out of date.
Advice and comments are welcome.
Side note: It would be nice to have Ubuntu's getlibs utility, which was made for this kind of thing.
Legal: I do not ensure that this will work properly on your system(s), and I make no promises to support it either.