The Terminal's Window menu bar lists Command-} (Select Next Tab) and Command-{ (Select Previous Tab) as keyboard shortcuts for cycling through Terminal tabs.

However, if you're used to Command-Left and -Right Arrows to cycle through Terminal windows, you can also use Command-Shift-Left or -Right Arrows to cycle through Terminal tabs.

[robg adds: And if you really don't like these shortcuts, you can redefine them in the Keyboard & Mouse System Preferences panel. Note that Command-Left and -Right Arrows still cycle between Terminal windows, though I don't actually see those listed in a menu. While testing this hint, I also found the Command-[ and Command-] will flip between Terminal windows, which also seems to be undocumented.]