I posted an entry to my blog yesterday (which if you subscribe to this, you might have seen). However, Google Reader showed the entire contents of my atom.xml file, ignoring the fact that I'd already marked as read the items previously. (And it will probably do so when the feed is updated with this entry, too.)
It turns out that Blogger hasn't changed; it's still sending the same IDs and timestamps on the entry as before, so it looks like it's Reader's fault. There's even a few discussions in the Google Reader newsgroups that has others have seen this as well, so something's clearly amis.
So apologies to readers who pick this up via Google Reader; it's not my fault, honest ...