If you are running displayLink chances are you have encountered a whole slew of issues when upgrading to Lion. But there is a work around. First, I set my energy saver settings to Performance rather then battery life which switches the video cards used. Then I installed the beta2 found on displaylink's site and rebooted my computer. This part fixes the blank login screen issue.
Now it's still a little buggy, the main one is that icons stick to the cursor. Putting your computer to sleep then waking it back up again fixes this issue.
I have a late 08 15" MBP 5,1 running 8gb RAM 2.53gHz 64bit lion 10.7 with two extra monitors. No issues so far.