Set the poster board on edge and place the projector at a distance that will produce an image approximately 18" to 24" wide. Then set your camera up on a tripod, with the axis of the lens as close to the axis of the projector as possible. Zoom the lens to fill the frame. After some tweaking of the white balance and EV settings, you can take high quality pictures of your pictures.
Fill up your memory card and import into iPhoto. There you can make adjustments and crop if necessary prior to burning as a group of slides, or make a slide show with all the trimmings. Using this method, you can import as many slides in an hour as you could with a scanner in a month of working in your spare time! This also works for your old 8mm movies.
[robg adds: I used a similar technique to convert old printed photos for use in an iDVD project. I set the photos on the ground, then put the camera on a tripod and shot down at the photos. The quality was more than sufficient for TV viewing, and I shot literally hundreds of images. Doing the same via the scanner would have been much more work.]

