This is something which I used to demo a lot when doing SharePoint Portal Server 2003 search demos (with Office 2003) and it still works with SharePoint 2007.
Starting from Office 2003 you have a research task pane in Office which allows you to search for information in a number of references (See How to Research Office training section) such as an English thesaurues, Encarta, etc ...
You can however easily integrate SharePoint search into this same research task pane. Just click Research Options > Add Services and next add a the URL to the search webservice of your SharePoint Server ... this will look something like this http://[servername]/_vti_bin/search.asmx.
The only thing which is a little different in Office 2007 is how you can access the research task pane:
- Word 2007, Excel 2007 and Powerpoint 2007: take a look at the Review tab - here you will see the Research option
- Outlook 2007: when composing a mail message - right click and select Look up - this will also open the Research Task Pane