There are a few reasons for concerns when it comes to getting surges of traffic. Now this applies from Digg, StumbleUpon, Netscape and other traffic sources.
First, feel free to read our write up named Is StumbleUpon Considered Auto-Surf Traffic & Against AdSense TOS? In that article, we pretty much determine that StumbleUpon traffic is not against AdSense terms of service.
The same logic applied to that argument is applied here with Digg. Digg is not an automated traffic generation tool. It is not a network that was created simply to create traffic. So in Digg's case, I doubt you will get banned from Google AdSense for getting on the front page of Digg.
In addition, many sites with AdSense have hit the front page of Digg numerous times are were not banned because of it.
One day of being hit by Digg doesn't matter, but if the ad supplier (in most cases Google Adsense) evaluates the traffic to show that your page is not as worthwhile because your CTR has dropped, you may be smart priced long after the diggers have gone.
Will Google know it is coming from Digg? Probably, so yes, this may not be a huge issue, but we don't know for sure.
Forum discussion at DigitalPoint Forums.