The first problem I encountered after the upgrade was that hp-toolbox insisted that it can't communicate with my printer (HP OfficeJet 5510 all-in-one). It took some futzing around to discover that the printer was accessible if I ran the program as root with gksu -u root hp-toolbox. Annoying.
By this time I managed to remove the fax printing queue from CUPS. Never mind, I told myself - just run hp-setup, follow the wizard's instructions and it'll be good to go. But here I got another surprise - hp-setup created a printer queue, but failed to create a fax queue, complaining that it could not find the HPLIP fax PPD file.
I futzed around a bit more, trying to manually add a fax queue from within the CUPS web interface. But hp-sendfax reported an error when I tried to use it (I later realized that I selected the printer PPD file instead of the fax PPD file - simply because there was no fax PPD file to select).
I finally decided to go over the hplip package bug page on the Debian Bug Tracking System. I then realized that I was actually hit by two bugs:
Bug #452454: to use the printer the user must be a member of the scanner group.
Bug #454341: the path to the PPD files is wrong in /etc/hp/hplip.conf, and should be set like this:
The fax works now. Dang.