Asking on behalf of a friend.
His 2011 MacBook Pro (no Thunderbolt) was running Snow Leopard and was being connected to a Dell monitor in the standard way.
He upgraded to Lion and now the MacBook cannot detect the display.
What can we do to detect the display (my Snow Leopard MacBook still detects the display)?
The MacBook was taken into an Apple retail store with the monitor. It was hooked up and obviously didn't work, It was then plugged into my 2006 MacBook, and it worked.
The chap in the shop was very confused.
We then looked at the cables. We used another cable and dvi-mini dvi connector. This changed nothing.
We have also done a fresh install of Lion.
So as for now the problem is unsolved.
As soon as it is fixed I will post the answer.