Hi!
If you want it simple, can't you just create a view filtered on Last Updated, showing only threads that have been updated within the last X days? Or, that might not be enough?
I suggest, just like Rob Creamer, to create a calculated column based on Last Updated and add a symbol or color coded green text saying something like "Updated!" if the thread is updated within the last couple of days. More info about color coded columns here (really great articles!):
http://pathtosharepoint.wordpress.com/2009/04/21/color-coding-formula-generator/http://pathtosharepoint.wordpress.com/2008/09/01/using-calculated-columns-to-write-html/Another way - and this may not the best solution - but what about creating a simple workflow in SharePoint Designer to add in example a new announcement whenever a a reply is created? the next step is to create a view in the "new replies announcement list" filtered to just show new replies fron the last X days. Then add the list web part to the discussion page. As I said, it might not be the best solution, but it is just an idea...
Good luck!!
Jenny
jennyfi