A couple of active Google Reader users have reported at a Google Groups thread that someone or something is injecting spam into their daily reading. It is really hard to determine how this is being done at this point (i.e. the reports are really new). Here are two of those reportS:

How are these people's blogs showing up in my reader that I haven't added to my subscriptions? Lots of Ads or people I just don't know.

I am having a similar problem. In my iGoogle homepage where I have a widget (taken from Google reader I think?) allows me see my feeds. I am seeing a feed from something called "best pics around." This now is flooding the widget so that i no longer see any of my other feeds. However, when I go to my Reader there is no sign of this feed and I see my other feeds as normal but therefore I am unable to unsubscribe to this feed. I have no Friends shared items and i am logged in to my account.

Google Reader Guide asks a few questions for the first user. He/she asks if she is on a public computer and making sure that you are indeed logged into your account and not someone elses. Maybe he/she is seeing items from the "Friends' shared items" that was launched recently? But I don't think this applies to any of those suggestions. The second report came in after and I doubt it was the same case.

I personally tested my account and it was clean of pictures and spam.

But what are other possibilities?

Someone hacked into their accounts and added the content

Their computer was infected and injected with this content

They visited a page that somehow automatically added subscriptions to their Google Reader accounts

Google Reader was indeed spammed somehow?

Forum discussion at Google Groups.