Would love this. Option #3 from http://feedback.textasticapp.com/topic/7436-dropbox-support/ is my favourite :)

Just a little asterisk badge would be perfect, one color for changed locally, another for changed remotely. If you want to go the extra mile, then allow the badges to be customised by the user :)

(although to be honest, if there was the option to automatically sync with dropbox it might make this feature redundant for me at least)