Do you just want to create a time stamp, or do you want the user to be able to enter in other comments? Depending on how you want to to do it you might be able to use publisher actions.
Good resource on publisher actions, it goes through all the steps to implement: http://help.salesforce.com/help/pdfs/en/actions_impl_guide.pdf
Good resource on publisher actions, it goes through all the steps to implement: http://help.salesforce.com/help/pdfs/en/actions_impl_guide.pdf