As you surmised, the two times a Touch can update the time are either when you sync data from iTunes, or use the Touch wirelessly. With the first option, the time is updated automatically; with the second, you can update it when you're online - and you already know how to change time zones pretty well, by now.
The iPhone can set the time zone automatically, since it's always has a network connection. It does seem like a rudimentary app to devise, and would prove useful. However, 10.6 is said to address this issue for some:
http://www.macosxhints.com/article.php?story=2009083013552497