Through the TextMate bundle framework, you get some nice collating of contexts, etc.
GTDAlt does rely on some proprietary syntax, but it’s pretty basic and easily parsable.
There is rudimentary support for iterating through items in an inbox.txt file to generate GTD items.
I created an applescript which will take the selected message in Mail.app and create a properly formated item, using the subject and message: url (you must have MailTags installed so the message: protocol is recognized).
This means I’ll get action items with links to specific messages in my GTD system.
In true GTD fashion, this should only be used for messages that take longer than 2 minutes to respond to. In reality, I need to get better at firing off quick responses to things (or liberally using the delete key). Otherwise, it’s likely I’ll use this to just procrastinate email items away into a black hole.