The following is based on information from Travis Wright:
KA's are displayed on the Self-Service Portal according to the Star rating on the KA. The higher the Star rating, the higher up the list the KA will appear on the Self-Service Portal.
Star ratings are averaged. If you enter a rating, and then you or someone else enters another rating, the final rating for the KA is the average of the two ratings along with any previous ratings that had been entered. (The ratings history can be inspected
on the Feedback tab of the KA form.)
If two or more KA's share the same Star rating, they will be displayed in the order in which the SQL Server database returns them.
Adam Bryer