MacBook Pro doesn't detect external display after Lion upgrade
This is from my own experience, I have macbook pro with lion and a dell monitor, I connect via mini displayport to VGA connector, if I connect displayport first to my macbook and then monitor's vga to displayport to VGA, it won't work, I don't know why but maybe some internal implementation detail, just go in this order


switch on both devices
connect monitor's VGA cable to your mini displayport cable,
connect mini mini displayport  to you macbook.
wait for a few seconds, it works every time.