Title field once changed can not be renamed back to 'Title' through UI. I had to make a change in the DB to forcefully rename the field but there is an easy solution also. Today while browsing the net, I found this interesting post that discusses in detail the cause and the solution for this problem:
http://sharepoint.microsoft.com/blogs/fromthefield/Lists/Posts/Post.aspx?ID=15
Title is a reserved word and once changed, can only be renamed back to 'Title' through programming.
ssa