I don’t have any slide files to test with but I put a black border on a 300dpi scanned photo. Then used Photoshop Elements 4.0’s “IMAGE > DIVIDE SCANNED PHOTOS” and it cropped off the black perfectly. This worked with a black border around the photo or with black borders on the right side and bottom only. The Divide function works with white or black backgrounds, on one or multiple photos. If you have multiple slides in one image, it should straighten and crop all for you as long at there is sufficient border around each slide. Kicker is that there are no automate commands in PSE4.0 so you would have to do it manually but it sure beats straightening and cropping by hand.
You can download a fully functional demo copy from the Adobe web site. If you try it, do let us know if it works for slides.
Morris