You're in luck: I use firefox and yes, firebug is up and running... and I didn't need that screencast either. ;)
Sooo, let's dig into it...
As for what ajax returns: something I would call "a json-ed array with empty positions". Therefore, I guess the "undefined" is a logical return we're seeing.
Trying to narrow it down yesterday, I detected the plugin does not have a hard time doing an "initial run" on new images (as they work), but as soon as a "rerun" occurs, even those working images render non-working, returning the same "undefined" results.
My best guess - for what it's worth - is that the plugin fails parsing the META of the images somehow. Probable a de-json thingy, where some data get's lost due to a parsing-problem. Why? Because in case of "undefined", none of the "undefined" images get scaled and the meta stored back in the database kills thumbnail usage by only returning the original, full-size image later on.
Love me or hate me, but I created a fresh install on my home server... created some posts, adding post-thumbnails to it. First run: all ok... 2nd run: all "undefined", post_thumbnails return original images, not resized ones.
Wrapping it up:
-> Create a post, add a post-thumbnail to it. Works.
-> Run the plugin. Seems to work fine...
-> Checking the post... post_thumbnail(); returns the original size image, no thumb (although the thumbnail itself exists on the server)
-> Rerun the plugin again... the "undefined" message shows up AND (as before) post_thumbnail(); returns the original size image, no thumb (although the thumbnail itself exists on the server).
I might be wrong - but as said: my best guess is that there's something wrong while the plugin parses the post-image-meta for the first time. When the plugin writes it back to the database - the thumbnail info is allready lost, so the original, full-size image data gets stored... but nothing else. Reruns of the plugin seem to be a quite good proof of that.
What you should know: I recently (last week as far as I remember) updated the plugin and I updated wordpress to 3.0.2 two days ago.
Before that, all worked. Since I can't find anything buggy in wordpress itself that can reproduce the error, I think the answer might be found in the difference between your previous plugin version and the current one (which allready seems to be supporting the upcoming WP 3.1). If I were in a devteam with you, I would simply ask: "what did you change"?
Wow, long message. Hope it helps you. If you need any more infos, please feel invited to contact me. I will try to do my best to provide useable feedback. (Please feel invited to drop an e-mail to "e-sushi" at "gmx.net" or simply find the rest of my contact info at e-sushi.net)