You might want to check out the posts here - http://forums.microsoft.com/TechNet/ShowPost.aspx?PostID=3416564&SiteID=17 - as they may help with one suggestion in there. Additionally, you should try running sp_helpdistributor to see if this database still recognizes that it has a distributor associated with it.
No concrete answer but it might give you a start in seeing what is going on.