I heard about an application for Android that sounded very useful and I thought would be possible on the N900.

CallTracker apparently creates events in your calendar when you make calls. This would be very valuable for me as I spend a lot of my day on the phone. Here's how I see it working:

1. I make or receive a call
2. When complete, a calendar entry is created from the time the call started until the time it ended and its title is Outbound/Inbound call to/from <insert phone number or contact name>

I could then sync that with my exchange calendar and create notes / track my time.

I think this would be very valuable, what do you think?

Sounds like a good idea - have you looked at the Extended Call Log app in Extras? Maybe worth speaking to to developer of that to see if anything could be integrated into or exported from that with a bit of work?