Hasin Hayder shares a method for "hacking" the Slideshare.net service to be able to download whatever slideshow you'd like:

I just spent some time today analyzing the HTTP requests made by the player available in slideshare using LiveHTTPHeader extension and found a way to download any presentation you want! There are many slides available in slideshare where you see that "Download not available" - But as long you have some knowledge on web-scrapping, that doesn’t matter at all.

He includes the code that you can point at a slideshow's URL and let run. It works through each of the pages of the presentation and pulls out the URL of it and echoes it back out.