I had a drastic drop in traffic between 9/6-26/11, but I can't correlate it with participating in the HP Ad Program which I had been enrolled in for several months before this drop occurred. I was told by various people that Google was probably tweaking its algorithms and that my traffic would return soon.
Wesman's response that "Google is testing your subdomain and their search results" is an intriguing one since I have no idea how Google tweaks or tests its algorithms, but I would agree that this is probably what you are experiencing. I was really bent out of shape for a good three weeks, wondering what I had done wrong, and traffic slowly began returning and I'm now getting more than I did prior to 9/6. So yes, hang in there and you should see an improvement soon.
By the way, Google ads are still being displayed on your hubs (if you thought Google might be mad at you for switching to the HP Ad Program). You still have a couple of ad spaces devoted exclusively to Google ads, and when the HP Ad Program can't find a higher performing ad, it will place Google ads in their ad slots.
Hope that didn't totally confuse you. If you want more info on how the HP Ad Program works, check out the hub I wrote about it.