could also apply to microblogging as well:

http://www.readwriteweb.com/archives/akibot_an_enterprise_twitter_clone_infused_with_ai.php

A somewhat different take: 
http://webworkerdaily.com/2009/03/02/ccbetty-offers-your-own-personal-email-assistant/

Another use case is time tracking, which could also include outgoing 
email prompts for cost objects, which users could then reply to with 
their hours.

I Want Sandy is (was) a good example of this sort of tech. Attaching 
the user guide and here are some related links:

http://code.google.com/p/sandysback/

http://sandysback.blogspot.com/

http://getsatisfaction.com/iwantsandy/topics/what_is_an_alternate_service_to_i_want_sandy



This is another service that does NLP:

http://www.readwriteweb.com/archives/deadline_task_management.php

http://www.readwriteweb.com/archives/email_still_matters_wordpressc.php

Create a script that can be used to receive email to multiple 
addresses for creating information. The address the message is sent to 
should be inferred from the email so that a single alias can handle 
messages for a whole Horde installation. Messages should be directed 
to a specific application via keywords and probably a survey of API 
methods to see what applications are available.



The address to send emails to should be configurable to some extent; 
possibly with a random string for minimal privacy. In any case 
destructive actions should not happen as a result of email processing.



Example use cases:



- send an email marked "note this" to create a note in mnemo

- send an email marked "schedule this" to create an event

- send an email with a vcard attachment to create a contact, or a 
vevent attachment to create an event

- send an email with an image attachment to upload it to ansel

- send an email to jonah to create a blog post