If you installed the latest software updates for your laptop, you should be running Snow Leopard 10.6.7.
Here's a tip from Keith Mountin at Apple regarding display quality when connecting your laptop to a projector:
Dark video when connected to a video projector with OS X 10.6 Snow Leopard
Some of you may have noticed that OS X 10.6 Snow Leopard, produce markedly darker pictures when connected to a projector -- especially when trying to play back video. This issue has been addressed in the 10.6.7 update. If you have been struggling with the dark video issues, try out the 10.6.7 update to see if that improves the situation.
Below are instructions on how to setup a custom profile if you not using OS X 10.6.7.
10.6 Snow Leopard shifted the default gamma value from 1.8 to 2.2. A gamma of 2.2 is what Windows machines have been using. This may impact how your image looks when hooked to a projector or external display. For more info, read this article:
In many cases, running through the calibration tool that you will find in the Display preferences will make things much better. If it doesn't, you may want to try this technique:
In the Display's preference pane, click on the Color button and then uncheck the box that says "Show profiles for this display only". That should give you a number of different choices. Try the one labeled "Adobe RGB (1998)"
Keep in mind, by choosing that, the colors may not be accurate, but the overall brightness of the image is greater.