<p>I need to get an appointment into someone's Outlook calendar based on requests from their employees. The application runs in SharePoint (WSS 3.0). My first impressions are to use iCal or send meeting requests, but I haven't done either before &amp; I'm looking for a very quick &amp; easy way to get it done. Any ideas?</p>

Sorry, there's no easy answer that I know of or can find.  I'm sure you can do via code but not easily within the SharePoint point-and-click interface.
From
this thread
it seems writing an event handler on a Calendar list would be the most effective way.
So from a 10,000 ft perspective, you'd build a system or workflow in Sharepoint that results in creating calendar entries in a SharePoint calendar.  Then have an event listener whose job it is to create iCal emails when new items are created in that SP calendar.
Brian Wilson (not the Beach Boy) has a few blog posts on
getting started with event handlers in SP