I'm running confluence 3.3. I just went (20100814.1525) to the plugin repository to install the community bubbles plugin and it downloaded/installed v1.6. I then went and tried to use it, ran into the NoClassDefFoundError: edu/emory/mathcs/backport/java/util/Collections bug and saw the forum post here saying it was fixed in 1.6.1.
Then I went back and looked at what version I had installed - 1.6 - and resolved the problem by manually downloading the v1.6.2 version and installing that.
Just FYI - something like tarfu going on with your repository version. It shows 1.6.2 as the latest but won't let you upgrade past 1.6.
Thanks for a great plugin!