Kit's Emailed Document Library / Lists
It have been some time since I've done this poll. There isn't a lot of responses, probably lack of interest in this area. I actually wanted to keep the code to myself, but I may just put it to codeplex, not sure yet. Anyway, this is what I did.
Basically, I've created a process of filing documents, and items into my personal KM. It is not a one email filing, but a series of emails that needs to be sent, if you do not have the template. That helps users not to remember the format of the email, and also prevent a chatty interface.
For a first time user, he / she will be sending a blank email to the document library / list, with the subject [Request]

After sending out the email, the user will get a reply from SharePoint, asking which content type he / she wants to file as.

As you can see, the list of content types configured for the document library / list are listed below the field. User just need to hit "Reply", and copy and paste the content type he is interested in filing against, and send it out.

SharePoint will then send a template email for end user to start the actual filing. This template can be kept for future filing, avoiding the first two steps.

The user just need to change the subject, as the system will use the subject as the title, attach the document, fill in the meta data below and send it out.
I still need to add support for multivalues, and user fields. Once that is done (with no timeline in sight), I'll consider publishing it in codeplex.
Comments are greatly welcomed.