Yeah, it's a browser-dependent thing. IE tends to be the most egregious offender in the unintended caching thing, as it tends to ignore nocache directives (but not, as you noticed, consistently).

The other option to consider would be to do this through an Ajax call: Set up a div to hold the updated info, and have the button action do an XHR to update the div. I would suggest not writing the code directly, but using a pre-existing library (jquery and yui are relatively lightweight, I'm partial to prototype, my JS guy from a previous job swore by mootools, it's all religion ultimately). Once you start doing the Ajax-y stuff, you discover that the old way of doing things is just painful.