I have noticed this as well. When I click and open the 'Back" item to see the list of pages I can go back to, I now occasionally see that my first link 'back' is the following:

http://pagead2.googlesyndication.com/pa

If I select the previous page I was on from the list, it works fine. So, for me (IE6) user, this is a recent occurrence... don't know what changed.. perhaps the method Google uses to block popups is diferent now.