<p>It seems that IE6 ignores any form of cache invalidation sent via http headers, I've tried setting Pragma to No Cache and setting Cache Expiration to the current time, yet in IE6, hitting back will always pull up a cached version of a page I am working on.</p>

<p>Is there a specific HTTP Header that IE6 does listen too?</p>

Have you tried setting an ETag in the header?  They're a pretty reliable way to indicate that content has changed
w3c Spec
&
Wikipedia
Beyond that, a little more crude way is to append a random query string parameter to the request, such as the current unix timestamp.  As I said, crude, but then IE6 is not the most subtle of beasts