The result is that even if you can prove that someone else holds a dollar bill that was stolen from you at some point, you do not necessarily have the right to get it back.
"legal tender" is a strange concept. For example, £5, £10, £20 and £50 notes are not legal tender in Scotland. But I don't think most Scottish people consider those notes to be "more like a commodity than money."
I'm not sure what happens if stolen Euros or gold are accepted as payment. These aren't legal tender in the US, so if they are exempt from the aforementioned rule, then bitcoins might be as well.
The result is that even if you can prove that someone else holds a dollar bill that was stolen from you at some point, you do not necessarily have the right to get it back.
See also the Wikipedia article on the principle of "Nemo dat quod non habet" ("no one gives what he doesn't have"): http://en.wikipedia.org/wiki/Nemo_dat_quod_non_habet