I accidentally split a tabbed terminal up into two windows and I can't figure out how to join it again. Google tells me that I should drag it until it gets translucent on some other version of OS X but that doesn't seem to be working.
You have to select View » Show Tab Bar if it's not shown (possible if your window has only a single tab).
Then drag all tab headers (just below the window title bar) straight up or down — not sideways — to initiate a drag of a tab across window bounds. Now you get the translucent "drag visual".
Drag this to the other window, and the dragged tab will attach.
Alternatively, you can always select Window » Merge All Windows from the menu.