A couple of months ago I was at the imperial war museum in Duxford and while looking around Concorde there was this discussion taking place.

Basically one of the Duxford volunteers looking after Concorde said that in France (can't remember where) they are trying to keep a Concorde in airworthy condition and are maintaining it and hope to do some high speed taxiway tests on it.

But that they have estimated that the process to get it certified to fly would cost the most amount of money and as such would cost around 1 Billion Euros!

Obviously I have no clue how accurate this is, but it was an interesting discussion.