A friend who talked to the band at the Stockholm show said that they were planning on co-releasing, or nearly co-releasing the DVD along with the new album. Regardless, the DVD has been effectively done (minus the interviews and bonus material, some of which had to wait for the band to return to Sweden from the US/Opeth tour) for a long time now.
Hopefully the December thing is a worst-case prediction, and they do release it this summer.