cgrantski - 2:51 am on May 8, 2012 (gmt 0)
You're right about how GA does it.
Some people write a script in the page code that fires the tracking tag every few seconds, adding a parameter or something similar to keep track of the number of seconds. With Google Analytics I'm not sure the analysis would be all that easy; I have never used GA with this method. I use WebTrends which has more power when dealing with parameters that have numeric values.
It will throw off your page count. In WebTrends, I filter these out of the main profile and have a separate profile just for this landing page timer. I'm not sure how you'd do it, or if you can do it, in GA. Maybe somebody else will have an idea.