syncevolution works with tasks together with the N9 and owncloud 5.

The backend is CalDAVTodo. You can have one todo-entry in syncevolution for each available calendar.

You have to use the newest syncevolution version 1.3

https://openrepos.net/content/holgern/syncevolution
or
http://people.debian.org/~ovek/harmattan/

You can find more infos here: http://www.nrtm.org/index.php/2013/01/09/4144/

This site is in german, but you can use google translate. I was able to configure my N9 using this site.

The following site was also usefull:
http://tanghus.net/2012/07/syncing-y...oud-an-update/