I was getting this message when trying to use a script to back up my database: "BACKUP LOG cannot be performed because there is no current database backup". My searching lead me to this link:http://forums.microsoft.com/MSDN/ShowPost.aspx?PostID=243166&SiteID=1. where someone else was having the same issue.
The problem happens when you change the database to simple to truncate the log, then back to full recovery. You have to do a manual full backup before you can do a log back up again.
I had gone to simple recovery in an attempt to shrink a log file that was not being maintained.