As a follow on...the issue with the latest version of jquery.form.js is that it requires you to put your JSON code in textarea tags if you're uploading files (this uses iframes and they're saying the textarea tags make that work better) like this:
<textarea> ...JSON goes here... </textarea>
If your server code doesn't wrap your JSON in the textarea, it just dies when it hits the client browser. Wrap it in textarea, and it works.
The good news is that it appears that wrapping your JSON in the textarea still works with the old version of jquery.form.js that is delivered via WP. So, if you update your plugin to be compatible with the new version, it should still work with the old version as well...at least my testing has done so. I'd love to know if you find out different!
Cheers to all!