I have been using VNC to connect from my Ubuntu box to an OS X box. On the Ubuntu side, I was using vinagre, which is the standard "Remote Desktop Viewer" application. On the Mac side, I was using standard desktop sharing.
Things were working for months, but they stopped working for me the other day. I could connect just fine, but I could only see a black screen. Apparently, this is a known issue.
Switching to Remmina (sudo apt-get install remmina) fixed the problem.