If you're hosting your blog on wordpress.com and are embedding video in your blog post, don't expect for it to show up in Google Reader.
That's what Google employees confirm in a Google Groups thread. According to Mihai, a Google rep, there's a whitelist that sites need to be added to in order for their embedded videos to actually play within Google Reader.
So in case you were wondering, apparently you need to request to be whitelisted. The policy isn't exactly clear on how you get your site added to the whitelist, nor is it indicated whether this whitelist is publicly accessible. I suppose that just means that you need to be on top of Google Groups (for now) in order for Google to consider your site whitelisted.
(Surely, there's got to be a better way...)
Forum discussion continues at Google Groups.