I agree that the Fed PD would be a great internship for you, if that's the direction that you want to go in. However, if you are wanting to go into the prosecution side of thing, this can potentially work against you. Furthermore, if this is the case, you would be better off finding a internship with the DA, since this would be more applicable to your desired field, and you would get a lot more valuable real world experience.
Given that the OP was dated Sept of 2009, I suspect that this ship has sailed.
As a general statement I doubt that interning on the defense side of the isle would cause any real harm to the career plans for someone anticipating working on prosecution, and may actually provide some useful insight for further down the road.