Beth,
a few comments on TSA Locks and securing luggage.
As you'll need to secure your luggage when leaving the U.S., it would be a good idea to use TSA Locks, in case the baggage screeners decide to check your bags. Some models of TSA Locks are equipped with a small "flag" that indicates if the Lock was opened using the special TSA tool (that way you know).
Although the TSA only exists in the U.S., I think it's likley that European airports will also have the ability to open the Locks without cutting them off. Baggage screening & airport security are "global", so it wouldn't make sense that only U.S. authorities could open them.
Using nylon Cable Ties is certainly a practical suggestion. You could also use the numbered "one use" luggage tags, which are available in Rick's travel store (as they're numbered, these provide indication the bag was opened). I believe Magellans has these also.
Given the recent experience of the Australian girl, I always lock my Pack!