No this is not a post on anything SharePoint 2010 related.
I get asked this question, quite a bit… It usually goes something like: I have a SharePoint site and when the user clicks on a Word document, the file is shown in the Internet Explorer viewer and not in MS Word. I had to track this down once upon a time with good old SharePoint 2003 so I can usually answer that it is a setting on the desktop that determines whether or not the rich client app will launch. At a recent conference, an attendee pushed for a little more detail and it took me a while to find the KB article. Here it is:
As you can see there is an interactive way to set it through some very hidden settings screens as well as through the registry. The registry approach published via a group policy would be the way to go if a massive amount of users need to have the change made.