The Google AdSense blog announced the release of up to five channels per one instance of an ad unit.
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 thing to remember is that each channel will report individual ad impressions, but on the aggregate, if you have five of the same unit on the same page, it will show lower impressions then if you would sum up the five custom channels for that specific ad unit instance.
Forum discussion at WebmasterWorld.