The Mobiface blog has an interesting set of screenshots from a UI concept based around a timeline. Every event and interaction on the handset exists within a flat heirachy which can be explored purely by date or filtered by type (e.g. phone calls, messages, content).
It’s an interesting approach, enabling users to track back through their activities in a more human way, without the need to remember which application was used for a particular interaction. The flat heirachy may also be a more appropriate approach as mobile devices are used for more and more non-voice activities.
You can read more on the Mobiface site.