If we would choose SWF, this would completely replace the current search box. If we choose JS, this would only be an extension to the current search box. This means users without SWF can't see the search box at all, while users without JS can use the search box, they just don't get instant results. Think about users in a limited situation: sometimes SSH and Lynx is all you have, when you need to search for the correct syntax of a function.
As I have said, it would not replace the search box, but would add another search box (which we had for years BTW at the top of the sidebar). But I am absolutely behind a JS solution :) See the compression solution by Mitja instead of argueing on SWF/JS :)
Goba