Tracking data usage on Android

I haven’t talked here about my new toy: an HTC Magic smartphone which runs the Google Android operating system (version 1.6 for now).  I’ve had it for almost two months and I love what it can do.  For me, it’s better than an iPhone, and it was so much cheaper to boot!

I have one nagging problem: I want to be able to track my data usage at an application level.  There are many apps which will track usage by day, and I can get a good picture of how much data I am using, but not why I’m using it.  Surely this isn’t a new need.  Can someone please point me towards an app that’ll do this?  If there’s an app for this, it’s not easily found in the Android Market or on the web using numerous granular Google searches.