That message really surprised me, because StumbleUpon is a legit source of traffic. Much like Digg, Delicious, Reddit, etc. - StumbleUpon is a social source of traffic. So I asked the publisher to email Google back for clarification on if it was StumbleUpon.
They replied to him stating nothing:
Thanks for your email. It is our policy not to endorse or support any specific product at this time. We're therefore not able to comment specifically on the program you've noted.
However, we recommend that you read through the AdSense Terms and Conditions (https://www.google.com/adsense/terms ) to ensure that your software it is not violation of any of our policies.
Thanks for your cooperation and understanding.
Now, we all know StumbleUpon can send a nice amount of traffic to your site in a day. Heck, go read Danny's write up on StumbleUpon - it will open your eyes.
Other AdSense publishers say they get a lot of traffic from StumbleUpon and they never received an auto-surf warning from Google. So at this time, I suspect that StumbleUpon traffic is OK with Google, but we have no official confirmation as of yet.
Forum discussion at DigitalPoint Forums.
Update: I got a tip from Gabs on an email from Google AdSense specifically asking this question:
Thanks for writing in. I'm not sure if I'm understanding your question correctly, but advertising on StumbleUpon.com shouldn't be a problem. You're more than welcome to provide us with more details to clarify the situation.
For additional questions, I'd encourage you to visit the AdSense Help Center (http://www.google.com/adsense_help), our complete resource center for all AdSense topics. Alternatively, feel free to post your question on the forum just for AdSense publishers: the AdSense Help Group (http://groups.google.com/group/adsense-help).
Max The Google AdSense Team
So I guess my assumption was correct, keep on Stumbling.