I once wrote to a website in the same genre as mine, asking if they sell ad space. The website owner replied that they do, but not to competitors. There's the problem with AdSense meetups - too many people are stuck in old-school business models, and feel like other publishers are in their way. Think about it - a website visitor can visit more than one website! This is not like two laundromats next to each other where a customer will choose only one. There are really no true competitors in an AdSense business model. People can click on ads at my site as well as theirs. Plus they could have earned some revenue from their "competitor."
I get the cold shoulder from others in my niche as well.
So if there was any sort of meetup, I don't think there would be any worthwhile discussion of good ideas. I think people would be there just to scope out the "competition."
The best tips and ideas seem to come from anonymous forums like this one, where we don't really know what niche the other website owners are in. We all sell "widgets."