Switching the Crash Test Dummy to Twenty Eleven... Done. Huh, it's doing the same thing.
Okay, I see what's happening. My post has the image saved with the http:// while before my old theme was showing the thumbnail in the excerpt. That thumbnail was dynamic and was getting https://.
Just for giggles I used the search and replace plugin and replaced all of my blog URLs with the https:// version (it is a crash test dummy after all).
That, with also setting the WordPress URL and Site URL to the https, made almost everything on my blog SSL based. Internet Explorer still complained because not quite 100% of all the URLs were SSL based but when I told it not to load the unencrypted URLs I couldn't notice anything missing.
Not sure that counts as a bug exactly, but if you have a work around that's probably good.