I decided to create an AppleScript the would take specially-formatted emails that arrive in Mail.app and convert them into tasks within Things. The subject line becomes the task name, and the content of the email becomes the notes associated with the task. I detailed the full process on my blog (with screenshots), but here's the executive summary version:
- Create this AppleScript, and save it to /Library » Scripts » Mail Scripts as things.scpt:
- Add a rule named Things to Mail.app to run the script when a coded email arrives. I personally chose to add an @ symbol to the beginning of the subject line to let Mail know to convert it in to a task. You can choose any symbol or criteria in your rule, just as long as it runs the script. I also chose to mark it as read and the script moves it to the trash since I don't need these tasks cluttering up my inbox.
[robg adds: I've recreated the code here in case the original ever vanishes, but the best source is the linked blog entry -- any updates to the code will be found there, not here. I haven't tested this one.]

