We don't have any first-party solutions for Symbian, but a third-party developer has created a Todoist-based app (using our API) for this system: http://éncoder.dk/%C3%A9doist/
Thank you very much!
Are you planning to create first party solution?
We will consider porting our mobile apps to other systems in the future. You're welcome to vote for this request in our Votebox: http://todoist.com/Vote/showProposal/205/