I am writing a title formatter for display suite, but title isn't a 'CCK' type. Can anyone help me out with how to define the formatter ?
Since title is a property, not a field, you cannot use a field formatter.
You'll need to take another approach to solve this problem, for example override the node template file, implement a hook_preprocess function, or implement hook_node_view.
edit: There are lots of discussions about properties versus fields. Here are some links to get you started with some background about why title is still a property, as well as some related modules.
Note: I have not used all these modules and do not endorse them in any way, and don't claim that they address your question. This is just a short listing for your edification: