ScheduleOnce, the service that helps , has new(ish) tool that lets users access its service without leaving the Google Calendar interface.
Using it requires installing a small add-on that currently works only in Firefox and IE with IE7Pro installed. Users then get a new ScehduleOnce menu in the left side of Calendar, either through Google Apps, or in the public version of Google Calendar. The control widget lets them see a list of meetings they have scheduled, along with the option to create a new one as if they were doing so back on ScheduleOnce.
What's really interesting about this add-on is that as of this week, it could no longer require the download to work. On Tuesday. A widget that makes use of this could let users keep track of tentative meetings they've set up, however any scheduling may still require going back to ScheduleOnce's site.