Althought i agree with some of your assumptions. I do not agree that the drop is anyways remotely related to PR toolbar export job. This is most definately a paid link panalty. If you are selling links, you are ought to get hurt. You said, Of course, why not. Advertisement is fine in eyes of google but that should be purely from the perspective of making sales or getting traffic not for getting PR juice. If your advertisement is purely for traffic/sales, why not add a rel="nofollow" to links and tell google u r a honest advertiser? People keep whining about paid links penalty but never understand why google is doing this and what do they expect from you.
well if not, then ur intentions are not clear to google. It will be counted as paid link instead of a advertisement since u didnt add rel=nofollow
Why? Google is playing GOD: they make money And deside which sites can make money!! They RULE the internet! WAKE UP guys!!