Long story short:

I got a new HP printer, it's too new for the hplip version on SBo,
so I compiled the latest version I could find on HP's site,
but the thing wouldn't print, though it did install correctly.

I had do this to get it to work:

make install put its files where cups wasn't looking for them.
The cups logs said the hpps filter was missing.
A quick symlink solved the problem:

nathan:~/temp/ross$ ls -lah /usr/lib64/cups/filter/hpps 
lrwxrwxrwx 1 root root 25 Sep  7 09:31 /usr/lib64/cups/filter/hpps -> /usr/lib/cups/filter/hpps*