It's very simple to get books from Bookshare on the Kindle. At first I thought you had to do it through sending the file to kindle and having them convert the document, but it's MUCH easier than that...
1. Open the Bookshare book (which usually opens in a web browser)
2. Copy and paste the text you want into Microsoft Word
3. Save the document as nameofbook.txt (not as a .doc. ALSO if this is in the newest version of Word 07, it's going to save the document as a docx by default. THIS WILL NOT OPEN ON THE KINDLE.)
4. Once you have saved the document as a .txt, plug in the kindle. You'll see the kindle icon on the desktop, or in My Computer.
5. Open the kindle icon and open the documents folder
6. Copy and paste the book (the .txt version) into the documents folder of the kindle
7. Eject the kindle and it should be on there when you turn it on!