With Chrome 12.0.742.112, if I redirect with the following headers:
HTTP/1.1 302 Found Location: http://0.0.0.0:3000/files/download.zip Content-Type: text/h
I've fixed this…by simply opening a new tab.
Why it wasn't working I'm not entirely sure, but it could have something to do with how Chrome deals with multiple downloads on a page, perhaps it thought they were spam and just ignored them.