AdSense have added the ability to use multiple custom channels on the one ad unit.
I have to admit that when I first read it when I got up this morning that it made my head hurt a little but I can see it being a useful option for some publishers who are really into tracking their statistics. 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).”
I suspect that 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 as we speak.