I think the MTA->Maildir->Mailman route is odd; why not go MTA->Mailman directly, like, say, GNU Mailman, maildrop/procmail, or any other mail filter I've ever seen?
You can use it as a local MDA/"mail filter" as well. Mailman accepts messages via Standard Input.
I don't recommend this, because you have to load and unload Ruby every time a message comes in. While this may be okay if you only have a few messages coming in, it will quickly become unbearable if you have any sort of volume. This will be even worse if you loading the Rails environment, as it can take several seconds or several tens of seconds, depending on the app size.
Side Note: There are at least three different projects using the name maildrop, and dealing with email, and I figured out which one you were talking about through context very easily.
