[quote]The shift-tab key should produce the white-space character since the tab key is reserved by the browser and by operating systems for keyboard navigation to the next form component, and making the tab key produce the white-space character would obviously conflict with that.
[/quote] Not on all systems... And we don't really have many multi-field forms here to begin with. 
 - All programs I've personally excessively used either use it as spacer or autocomplete (latter mostly only for commandline/terminal/console-based programs, though the the terminal also uses it as spacer in some very specific instances). ...Excluding computer games, where it is often minimap, console-call, or some such unless I've mapped it to some other commonly-used function, since it is conveniently right next to WASD (though I use QWSD instead).