I'm running 3.0 beta2 with the Atahualpa 3.4.9 theme. I tried to install the Tierra Billboard Manager plugin, which I have successfully running on several WP 2.9.2 sites. I got this error:
Plugin could not be activated because it generated 222 characters of unexpected output.
Tierra Innovations created this plugin for WNET.org and it can be seen on various of their websites. I will report this problem to them as well.