In the gnome-tweak-tool, I clicked "Workspaces" on the left and then changed "Workspaces only on primary display" from "On" to "Off"
Maybe "Off" would be a better default, people have blogged about this setting: http://gregcor.com/2011/05/07/fix-dual-monitors-in-gnome-3-aka-my-workspaces-are-broken/ This is not a complete fix, in a couple of brief tests I observed that windows remain on the same workspaces when I suspend and undock the laptop, but I still observed problems such as the workspace switcher applet showing the thumbnail workspaces being the wrong size with big gaps around them
