The folks behind Zenlist announced their decision to close their website:

We will be shutting down service on 11/15 due to lack of interest, poor development progress on our end, and a general feeling that Tracks is not yet ready to be a hosted solution unless significant contributions are made. hosted the Tracks application, a software written in Ruby On Rails that supports the Getting Things Done methodology for self organization and task management.

I've been using Zenlist for some weeks, but recently reverted to using Backpack after repeated outages and downtimes of Zenlist. I can only use Web 2.0 applications that I really can rely on.

It seems that they finally gave up on getting Tracks to run smoothly -- or couldn't be bothered since the service apparently didn't quite get traction, anyway. On the other hand, there could very well be a connection between lack of interest and an unstable service.