Once WP7 has been out for awhile and people start hacking it they may find a way to fix this. In Windows Mobile there was away to change the delay in ending a call. If memory serves me correct is was 2 seconds, and you could take it down to a half second if you wanted to.
I wouldn't hold my breath - Windows Phone 7 has more in common with a Zune HD than it does with any previous Windows Mobile phone. Although clever people do tend to find a way to accomplish certain things, so who knows. But I think the days of registry hacking and whatnot are likely behind us (for better or worse).
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