I think it's a pretty neat idea to incorporate a measure of "karma" based on a user's recent activity across launchpad. I'm not quite sure what of the exact algorithm for calculating karma, but I've heard it's time-dependent. That is, more recent activity is awarded more points, but its point value seems to "fade" over time. As a result, a user's karma will fluctuate up and down over time.
I think it would also be nice for users to be able to see a timeline of their own karma record. It would be fun to see, over time, how a person's involvement in Ubuntu and other projects has increased or decreased over time. Also, it could be a motivator. i.e. if a person notices that their karma is slowly dropping because they haven't been as active lately, they'll be more inclined to answer questions, work on bugs, commit code, etc.
There is already a karma history page on each users' profile. This would be a great place to show the timeline graph. I'm imagining a 2-D line chart with karma plotted against time (time can be scaled to different resolutions), although other graphs may work as well.
Note: I've also opened an "idea" on brainstorm to gather more input from the community: http://