Don’t use the conditional query tags (is_page, is_post(), is_*) prior to init. In 3.0 and earlier, this merely failed quietly. In 3.1 this will produce a fatal error. We’ll workaround the fatal error prior to the release of 3.1, but in the meantime we’ll leave it to serve as a flag for improper use of the query conditionals. If you see a fatal error of the form “Call to a member function is_*() on a non-object” then a plugin or theme is using that function prior to init. See ticket 14729.