During my Washington, DC trip of a few weeks ago (to check out, and eventually accept a place at, Catholic University of America School of Law) I ended up chatting with some folks from the Air Force OSI. OSI's mission is most succinctly described as the Air Force's in-house FBI/CIA/Spooks. They have a much broader mission profile than the larger agencies, and, being small, they operate with a greater degree of independence. Sounds pretty interesting, and only reinforces my interest in some of the spookier careers out there. One of them had a bit of advice for me when I told him that I would be traveling Europe this summer: Take a Standard Form 86 with me and fill it out along the way.
Apparently the US Government is mighty particular about where you've traveled and how you account for your time while abroad if you are interested in working intelligence. SF-86 is the standard questionnaire for national security positions, and collects this as well as much more information. So little as a single day or country not disclosed can be grounds for immediate and irrevocable termination of candidacy.
Considering my and my brother's perennial interest with this sort of work, I think it's probably prudent to do exactly as the man says. Plus, once its filled out for this trip and for our previous trips (including Canada, Vatican, British Virgin Islands, and Berkeley) we'll have a complete list of everywhere we've been for ourselves, as well.
[Edit: 2/2/09 - That advice was right on the money. Though it's been my experience now with DoJ and State that low level (less than Top Secret) clearance doesn't demand the closest scrutiny, not having at least estimated information will leave some conspicuous holes.]