Does Adsense prohibit the use of StumbleUpon?

Is the Firefox add-on StumbleUpon considered an autosurf program and, thus, prohibited for use by Google Adsense?

Last week, I received the following email warning from Google about supposed invalid clicks or impressions in the site:

It has come to our attention that invalid clicks or impressions have
been generated on the Google ads on your site(s) through users of
third-party programs paid or provided with other incentives to visit
your site. Such programs may include, but are not limited to auto-surf,
pay-to-surf, pay-to-read, or pay-to-click sites.

I am not into traffic exchanges and the last time I used an autosurf to promote PinoyMoneyTalk.com was in mid-2006 so I wonder whether my use of StumbleUpon was the culprit for such warning.

I know Google won't divulge anything about the exact violation but I still tried my luck by replying with a question of whether StumbleUpon is prohibited in Adsense's TOS.

Google's reply:

Thanks for your response. While we're unable to comment on the compliance
of any specific tool or service, please note that AdSense participants are
solely responsible for verifying that any tools or software used in
conjunction with AdSense do not violate the AdSense Terms and Conditions.
As a reminder, any method that artificially generates clicks or
impressions is strictly prohibited, including automated click and
impression generating tools.

If, in the future, you're concerned about the activity on your site, feel
free to review your site's traffic logs for suspicious activity and notify
us of your findings at
https://www.google.com/support/adsense/bin/request.py?contact=invalid_clicks_contact.
This information can help us in resolving any issues, although as we've
noted in our Terms and Conditions, Google will use its sole discretion
when determining instances of invalid clicks.

In PinoyMoneyTalk.com's case, although traffic did surge a bit, the number of ad clicks did not rise significantly. So for now, it looks like it's still a guessing game with Adsense whether StumbleUpon is allowed or not.

I tried searching for discussions about this issue but found nothing substantial. The closest I got was this August 2006 thread in WebmasterWorld. The general consensus there was although StumbleUpon does cause a spike in traffic, it is not a cause of concern yet for Adsense publishers — unless it is responsible for a similar spike in CTRs.

My question, however: if Adsense disallows the use of StumbleUpon, why did I get the warning only now when I have been using StumbleUpon since October of last year? And what about other sites that I "stumble upon" in SU that still contain Adsense ads? Are they not getting penalized for supposed invalid clicks and impressions?

Hmmm…