I upgraded to 1.3.12. Download speed for all torrents was 0. Ports were open (it didn't seem that any of my preferences were changed). Each torrent shows a number of seeders and peers, so it's not an issue of Deluge not finding any peers.
I uninstalled 1.3.12 and re-installed 1.3.11. Immediately, all the torrents are downloading.
I'm on 64-bit Windows 7 Home Premium. Not sure what other information you might need to help diagnosis, so please let me know.