I've read through the entire thread, and I don't think anyone mentioned the "Cache-Control" header; The as bulletproof as you'll get solution would be with the follow chunk of headers:

Cache-control: no-cache
Pragma: no-cache
Expires: (time)
[download]

The time should be expressed in one of the formats mentioned in section 3.3.1 of RFC2068)

This should catch all variants of agents and intermediaries, however, I expect there are still a few old/badly behaved agents out there...

--
RatArsed