You'll see it in Console view of Firebug. Might have to refresh the page after going to console view and you may have to enable console in Firebug settings.

One could try to write their own widget but it is a shame that amazon hasn't done a better job providing one. The product preview thing came out in like 2006 and it still says "New" next to it. 

I'm curious have you actually made any significant revenue from amazon?  I've used them for years on joeaudette.com and on this site and made next to nothing. 

I'm not sure the popup product details is really going to help your conversion rate much.I think just writing personal recommendations snippets beneath the ads telling your readers why you recommend the book/product will help your conversion rate much more than the little popup. You could also use their iframe widgets to show more product detail.

Best,

Joe