I don't post over here much and I don't even own a Treo. I know -- very helpful so far
If you are going to be traveling internationally I would think a GSM Treo -- especially an unlocked one -- would be the best bet. That way you can purchase SIMs in the country that you are currently in for business which is much, much cheaper then using Cingular or T-Mobile's roaming prices. I am not too sure about how using data internationally would work ...
As far as Cingular/T-Mobile I agree HoFo is a great place to start. Although I did just read (Brighthand I believe) that the 680 is NOT going to be launched on T-Mobile and that the 750 is coming to Cingular next week (at least announced next week at CES).
If you have travel questions I think flyertalk.com is the best site for travel. They have a great travel and technology forum!
And congrats on the new job!