I found the following information in an old PowerPoint presentation that I created a few years ago. I do not have the original SAP document so take this for what it is worth. This information may have been revised after I put it in a presentation. The versions listed are the last supported version for each platform (v5.1, v6.1, v6.5, XI)
Business Objects 5.1.9 - Released: 24-Dec-04 - Tech Support EOL: 31-Dec-07
Business Objects 6.1b - Released: 15-Apr-04 - Tech Support EOL: 30-Jun-07
Business Objects 6.5.1 - Released: 15-Sep-04 - Tech Support EOL: 30-Jun-08
Business Objects XI - Released 16-Dec-04 - Tech Support EOL: 24-Mar-09