Could an app keep a trail of selected iPhone actions in a journal type style? Would it be possible for an app to be able to automatically keep a trail/log of iPhone actions? E.g., I could see all phone calls, text messages, emails and other actions I had taken in a day (all in one timeline/journal type display)? Ideally being able to configure the app to choose which actions I wanted to take a trail of?

Is there an app that does this now?
iOS applications run inside an individual sandbox, which prevents applications from accessing data etc. from another application. This also prevents a general "Activity Log" type application from working, unless Apple would provide an API for things like this.

Things may look different in the jailbreak world but even there the applications to be monitored would need to "publish" their actions somewhere.