So for awhile now I've been half-thinking in the back of my mind that I need to refresh myself on the HTML specs for input elements, since in Safari I wasn't able to tab through forms as intended. For example, when I'm logging into certain websites like Good-Tutorials, I want to just keep tabbing through the form fields, hit the checkbox, tap spacebar to check the checkbox and then hit return to submit the form. In other words, I want to be able to use forms without having to move my hand back to my mouse where possible.
I always thought this was some inherent problem with form fields (that checkboxes needed a separate attribute to determine the tab order or else you couldn't tab to them), but after looking for a different thing I stumbled on the Keyboard preference pane in Leopard. (I'm not sure if this is Leopard-specific at this point- if you can verify this, let me know.)
To enabling "proper" tabbing, go to the keyboard shortcuts sub-pane and on the bottom is a section for "Full Keyboard Access". Make sure "all controls" is selected, and reload your web pages and you should be able to tab as intended now. I checked and Firefox is prone to this too, so this option fixes the behavior in that (presumably in all browsers as well).