This morning I accidentally dragged an iTerm2 tab and created a split pane in one of my tabs. I searched for quite a while to find out how to move it back to its own tab. Finally, I found it at http://groups.google.com/group/iterm2-discuss/browse_thread/thread/920ff96f710a0a93:

You can already do this by holding cmd-shift-opt and dragging the pane to a
tab bar or away from any terminal window to create a new window.