I've stopped using Ecco Pro and Shadowplan, and I'm now using Toodledo.
I looked some other online systems, but Toodledo was the only one that had both state (well, folders) AND context. Since Toodledo is a web based task system was that I'd be unable to add to it when I wasn't in front of a computer. However, with the integrations to Twitter and Jott, I can SMS or even phone up and leave messages.
Once I found out there was an API as well, it was really only a matter of time before I'd find myself writing an adapter for it. And a command line client, in case you want to access it from a shell.
So here's my first Ruby gem.
If you're on a mac, you should be able to type 'sudo gem install toodledo' and then 'toodledo setup' and that should walk you through most of it.