Browsing my site in firefox I noticed a "read i.ytimg.com" in the status bar after the page had loaded in.
Being paranoid, I started scanning the page for links to "i.ytimg.com" to figure out if something was located in the source, but nothing. Then I checked the domain ytimg.com in dns stuffs domain finder and it pointed back to Google and YouTube. Having a number of YouTube videos embedded into some of my posts, I felt more at ease with this mysterious domain and message.
Then I did an ordinary search on the domain, and behold - apparently (many) others have had the same worries/experiences as me.
So apparently ytimg.com is used by Google for their YouTube service. Phew.