To enable tab navigation of all form elements in Safari 1.2, you'll need to open System Preferences and click on the Keyboard & Mouse icon. Then, in the Keyboard Shortcuts tab, check the "Full keyboard access" checkbox. After doing this, you'll be able to use your keyboard to truly navigate all form fields (which includes checkboxes, menus, et al).
You should note, however, that turning this option on enables keyboard access system-wide, which might be something you're not used to. Full keyboard access will allow you to tab between OS X interface objects (like buttons and such), and might alter your behaviour slightly.
It might be nice if in the future Safari had the keyboard access feature on its own without having to enable it system-wide, but in the meantime it's still a great feature.

