I had previously presented using Keynote with the VGA adapter on a projector without issue but today it failed. Luckily I had my laptop along, not being overly confident that everything would work out, and the laptop worked fine. A little sleuthing turned up that the resolution of the video out has to be 1024x768. Seeing as I was at a conference center with a huge probably old projector I did a quick check using my laptop -- yup, 800x600.
So word to the wise -- if you're using someone else's projector, check ahead on the resolution!
So word to the wise -- if you're using someone else's projector, check ahead on the resolution!