It will depend a lot on where you intend to travel. I currently own a separate phone for travel in Asia where GSM is used. Cheaper to buy a sim in the area you will be and purchase a "load" or prepaid time. Just came back from the Philippines and SIM was under USD$4. Bulk of expense is in the prepaid time. Unless you visit each area repeatedly, the SIM will normally expire and the number recycled back into the market. For reference, I purchased a Nokia 1100 brand new for USD$50. Basic model. Darn thing even has a flashlight on it. Charger is 110-240v.
Most domestic US SIM based phones are "locked" and can only be used with SIMs issued by the specific company. From my experience, Nokias/Ericssons tend to be the easiest to "unlock" which can be done by many small shops in SE Asia. Motorolas are hit/miss, while Samsungs could not be unlocked. Of course, YMMV. These should be tri- or quad- band phones. Should run about USD$10 for the service, most shops will only assess the charge if they are sucessful and a local SIM works. Extra advantage is that these phones normally come with universal input chargers, as most of these areas use 220v.
You can stick with your existing company if they allow global roaming (I had the service with T-Mobile) and you have a "world" phone, but you will get killed on the roaming charges. Learn to use the txt function, much cheaper. Japan has a totally different system (3G, I think, or something like that).