Sometime ago, I read somewhere on the web that there's a Google testing program in which the participating sites are eligible to try out new features. Did Google pick this sites by hand? If we would like to join, how do we apply?
I am not sure about testing for regular publishers, but watch to see what the Premium Service ads are doing - they got the different ad sized before it was released to the regular publishers. They might have had colored ads earlier as well, but I can't quite remember now. They now seem to be able to have more than one AdSense ad block on a page, so we could see this for regular publishers in the future too.
So once you get more than 20 million page views a month, you may get first dibs on cool new features ;)