The email thing is compelling, because there's no way I can move email tasks out of email (just too much work and shuffling around to make the physical move), but it's relatively easy to move things into email. The only problem is the interface... but maybe with a little procmail, some client extensions, and some scripts (e.g., a script to add a task, which needn't be as complicated as opening a new email typing my name into the address and all that)... that might work sufficiently.
This post seems to be the best summary of a labeling method for Thunderbird. I like that it uses labels even for things Thunderbird does natively (like deletion), because in my experience some operations (like delete or moving folders) require just a little bit more time to process (say, 0.5 seconds) that slows me down. Flagging and operating en masse after the processing is done will give that little speed boost that I know I'll appreciate.
todo.sh looks interesting, but I don't know how important I'd find some of its features. Specifically, managing a list through the command line seems a little crude. I think just being able to send messages into the system without using a mail client will be sufficient for me.