I have google.com blocked in adsense because I saw Google ads too frequently on my site but that isn't stopping a Google Chrome ad from appearing quite often. A look at the unit tells me it leads straight to the Google.com chrome page so it *should* be being blocked but it isn't being blocked at all.
Maybe you can't block them anymore, or maybe they're using a doubleclick URL, or maybe it's taking longer.
Personally, I might try taking an ad unit off for a while to see if that makes it go away (unless you only have one on a page now) I have a feeling those Chrome ads don't pay very much; they're probably filler, and you want the higher paying ads to show up if possible.
The actual url is google.com/chrome without redirect, at least that's what is displayed when I look at their flash file.
The behavior stopped, the ad is gone now, perhaps they had a temporary problem with their ad serving? Adsense also said it had issues today. There was an apology message for data being unavailable in my adsense dashboard at one point.