Do u have the habit of sending an email saying "please see the attachment" and forget to attach the file?
Gmail Labs has the solution, After logging to Gmail -> Settings-> Labs.
Go down and find the Lab "Forgotten Attachment Detector" .
Google says "Prevents you from accidentally sending messages without the relevantattachments. Prompts you if you mention attaching a file, but forgot todo so." .
I tested this and it couldnt detect the mail with body . "please see the attachment" , but it was able to catch the sentence "I have attached a file".
May be in future google might come up with more releiable mechanism to identify forgotten attachments. but its quite useful to enable this lab now.