Thanks for posting. Wow! That’s a cool Ubuntu setup - If you’d want to share the xorg.conf, that’d be great for others to see. And did you create a mechanism to switch the config between lid closed + DisplayLink 3-screen vs. lid open (and DisplayLink screens disabled) modes?
Now, on to the real question (Mac). OK - if yours is a new MacBook Air, then you’re running Lion (10.7) right? Lion unfortunately has taken some steps backward from Snow Leopard with the same DisplayLink USB graphcis drivers (especially in performance). And there have been some other behavior changes in Lion (neither positive or negative).
First thing to note is that Windows has a control panel setting for “what to do when the lid closes” that lets you determine whether the machine should sleep or stay awake, etc.
Mac, in contrast, avoids having any users settings for clamshell mode, and rather has chosen to have the behavior be automatic based on whether it detects the presence of laptop AC power, external display, and keyboard or mouse. If all three are present, the mac will automatically stay awake in clamshell mode.
And the specifics of the behavior have changed a little in Lion. You’ve probably already seen it, but here’s the Apple KB article that explains the automatic behaviors (including the differences from Leopard to Lion): http://support.apple.com/kb/ht3131
Now to the important DisplayLink USB graphics limitation - Mac OS (at least prior to Lion, and with Macbooks of that day) apparently did not recognize a USB graphics device as an external monitor. So if only USB displays were present, Mac wouldn’t enter clamshell mode.
So to get clamshell mode working, at least one of the displays had to be directly connected to the MacBook via DisplayPort or DVI (you can see posts of configs that have successfully worked at http://www.displaylink.org/forum/show…)
But from your description, it sounds like you’ve worked around that problem - you don’t have any external displays attached to the built-in DisplayPort port, but the Mac doesn’t go to sleep in clamshell mode - is that right? Have you had to use any special settings or power utilities to get to that point?
If you can let us know that, we’ll start honing in on the two problems: 1) why the macbook main screen doesn’t sleep 2) why the 3rd DisplayLink display doesn’t come on.
Thanks again for posting - these are tough but interesting uses!