Note: this hint involves editing a Nib file... if you don't know what that is, this hint may not be for you. You'll also need the Developer Tools / XCode installed.
In most Mac apps, Command-Shift and one of the arrow keys is used to highlight the text to the left (or right) of the cursor in a text field. However, Apple (for some reason) decided to override this in Safari, binding it to shifting between tabs. Personally, I would prefer that Command-Option and the arrow keys were used for that task. So I decided to try to edit the Nib file for Safari to make this work.
Quit Safari first, then open this Nib file in Interface Builder: /Applications -> Safari.app (control-click and select Show Package Contents) -> Contents -> Resources -> English.lproj -> MainMenu.nib. You may want to make a backup of it first.
Now simply click on Window, then double-click on the glyphs to the right of Select Next Tab. Do whatever key combination you want (I used Command-Option-Right Arrow); do likewise for Select Previous Tab (Command-Option-Left Arrow). Save the Nib, and close Interface Builder. The next time you open Safari, your change should be made, and text areas in it will act as they should.
[robg adds: Keep in mind that any future Safari updates may override these changes if they include a new Nib file...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20041128024426828