During plugin activation, WP 3.0 checks to see if the plugin generates any output. If any output is generated a warning is shown to the user. Generating output during activation is usually a good sign that the plugin has spaces before the opening php tag or that it generates warnings. This extra output can break feeds and cause “headers already sent” warnings. Please test your plugins against 3.0 and make sure your they do not generate unexpected output.

See tickets 13585 and 12089.