It works now! you will need to jump through a bunch of hoops though. especially if you use x86_64 Linux.

Update2: everything just works now.

1. Download the i386 flash tarball. extract it and copy libflashplayer.so to /usr/lib/chromium-browser/plugins.
2. You'll also need to install some other packages if you are on x86_64. These are the ones I needed; ymmv.
libXt.i586 PackageKit-gtk-module.i586 (probably don't need this but I got a message about it when flash player crashed)
3. Start chromium with these options: chromium-browser --enable-extensions --enable-plugins

I'm pretty close, I think. When I load a page that uses the flash plugin, I get a message stating chromium crashed (it doesn't. probably just a subprocess) and this message in console: 75c46662-5199-db0c-33c57d6e-1d2dbf27 is dumped. problem is probably an old nightly build of chromium or something, I am using it packaged from a repo

source: http://h3g3m0n.wordpress.com/2009/07/12/linux-chrome-flash-ext/

update: it works now. ignore parts that are striked out. scrolling with pages that have flash on them is really slow, though. the slowness is proportional to the number of flash elements on the page, which really sucks. they'll probably be optimizing things eventually though, plugins are still "partially implemented"