In Aperture 3.5, sharing previews with other Apple applications is now handled differently than in previous versions. Previews from the Aperture library are always available to iMovie, Safari, Mail, and so on, regardless of Aperture's settings. This means that the sharing preference has been renamed from "Share previews with iLife and iWork" to "Share XML with other applications.
New libraries created in Aperture 3.5 default to setting the XML sharing preference to "Never", but libraries created in Aperture 3.3 - 3.4.5 will retain their previous sharing settings. If you do not use Aperture media in any third-party applications, setting XML sharing to "Never" in existing libraries is best. This eliminates the need for Aperture to write periodically to an XML file which can cause a spinning wait cursor to appear as you work, or when you quit Aperture.
If you use previews from Aperture with third-party applications, then the XML sharing setting may still be set to "Always" or "When quitting Aperture".