If you want to search your Inbox, Calendar, and Tasks all at once, you can. To do this, you would first switch to the Folder List (click Folder List in the Navigation Pane) and use the Instant Search pane there.
Otherwise, search is restricted to one type of Outlook item at a time. For example, you can search for messages or you can search for items in your Calendar. The good news is that once you get familiar with the new searching tools in Outlook 2007, you'll use a similar Instant Search pane and techniques in each Outlook window including Calendar, Contacts, and Tasks.