Mmm... Well, I didn't place an add in my blog (besides the blogger add), but I have mentioned Google a lot, and I guess it could have worked for me.
I now have a PageRank of 5. I think it began when I mentioned Google in my blog (actually it was a just a little critique). Then I mentioned when I installed the Google toolbar.
But I think my PageRank got its boost when I wrote lots of comments on some Microsoft blogs.