Check out this article by Nigel Cheshire - quoting both Sam Guckenheimer and Joel Spolsky about whether or not measuring developer performance is a good thing or not. Both Sam and Joel say to stay clear away from doing this. Where do you stand? Personally, I say that we need to "professionalize" our industry a bit more before we can be effective at doing this. Tools like Team System make instrumentation of developers a lot easier, however I don't think we can clearly understand the inter-relationship of metrics on individual developers well enough yet. Nigel compares Sales people to Developers in this respect - asking why we can ask Sales people to hit metrics and not ask Developers to do the same.
I think we'll be there one day - however, I think the real way to measure developers is how they interact as a team - and from that measure the team by the results of that team - in this case the software they develop. Our industry needs to has a lot more maturing to do before we should focus on individual developer metrics, as there are likely hundreds of other aspects organizations can improve upon first that have greater value.