For the most part, you can always find a Google AdSense layout that you like and figure out how they've laid it out by viewing the source to the page that includes the advertisement.

You mention Digg.com and, yes, they do have an attractive banner advert on the top, as shown in this figure:

Popping open the source itself (which can be done with View --> Page Source... or similar, depending on your browser) and scrolling down a bit, we should be able to find the exact sequence of AdSense code that they're using at Digg.

Unfortunately, Digg is part of the Federated Media Network which means that they're not using the usual Google AdSense code to serve up their advertisement, but rather some sneaky FM code. :-)

Fortunately, I recognize what they're doing anyway, which is having just one text advert. You can do that too if someone buys the exclusive rights to your own AdSense space through the AdWords program.

The easiest way to achieve this effect, however, is to work with a graphical banner ad, so let's pop over to the AdSense site and check out how to work with graphical banner ads instead. The basic approach of using CSS to create the border around the ad block is the same in either case.

Log in to your AdSense account, then go to AdSense Setup --> Products --> AdSense for Content. You'll then want to choose "Ad Formats" which you can reach here: AdSense Ad Formats (you probably need to be logged in to see that page, actually). On the page are lots of different formats, including the one we want to see:

You can't select this, however, if you don't also specify that you want image-only or text and image ads as your Ad Unit:

This gets you part of the way there, but the additional fancy outline that Digg has is something special they've tweaked. You can probably attain the same by using a border within a DIV sector that wraps the Google block too, but it still won't be quite as attractive as the Digg ad block, which they've done in cooperation with Google.

Here's a sample of the CSS you could use, I'll just wrap the graphic shown earlier so you can see how it's done:

Not quite as attractive, but still, not too bad to look at. The code that I used was:

Hope that helps you out! Oh, and if you haven't signed up for AdSense yet and started monetizing your visitors, why not sign up for Google AdSense right now?