AdSense have added the ability to use multiple custom channels on the one ad unit. you’ll be able to add up to 5 custom channels to a specific instance of ad code.
I this could be an useful option for some publishers who are really into tracking their statistics. It’s a hard job to some publishers to update all their ads code for some big websites. But I think it could be useful to track at the same time the ad format and the section of the website were the ad is. The example AdSense use to explain it is probably the best:

“What’s the benefit of tracking with more than one custom channel? Well, multiple channels can be very useful when you want to track one ad unit across several different metrics simultaneously. For example, let’s say you run a sports website and you’ve placed a leaderboard at the top and bottom of every page. To track the performance of the ad placement, you’ve created two custom channels — ‘TopLeaderboard’ and ‘BottomLeaderboard’ — and regenerated your ad code appropriately.

But what if you also want to compare your football pages and your baseball pages at the same time? With multiple custom channels, this isn’t a problem. Just create two new custom channels called ‘FootballPages’ and ‘BaseballPages’, and add them to the appropriate ad units. Now your leaderboards will each be tagged with two custom channels that let you know which position they’re in (top or bottom), and the type of page on which they appear (football or baseball).”

The majority of AdSense publishers will never use this, but that a small number of publishers are busily adding multiple custom channels to their sites in the next days. But you have to keep in mind that ad units tagged with multiple custom channels will log impressions or clicks in each channel. As a result, you’ll see a higher number of impressions and clicks when you view your channel reports than when you view your aggregate reports.