Once you've (by accident of course) changed the name of the title column, there seems to be no way back.
You cannot rename the column back to Title because SharePoint tells you this is a reserved name for a column. There are some solutions on the web which require coding or database access. I found that by using the SharePoint Manager 2007 tool you can easily rename the column. No coding or SQL skills required.
Just browse to your site collection, open the Site Columns, look for your renamed column and change the name property back to Title.
Please note that you need to enable the 'full object model' mode:
