You can do this for specific sub forums, but I can't see how to do it for the whole forum. or for specific groups.
To try it out, place the core RSS module (Newsfeeds) on a page and then set it up using the XML feed from one of your subforums. You'll see a selection of latest posts in the RSS module.
To make it useful, you'll need to spend some time customising the RSS styles/ASCX/XSL files in order to provide a more useful display.
An alternative is the one by Peter Donker that is going to become the new core RSS module. Have a look for it on his Bring2mind site. By default his module looks awful, but it has better functionality than Newsfeeds and with some work you can make it look very good. Doing so will also teach you about scripting the XSL file to suit your purposes. I have made mine show subject/author/date in the popup and have of course made the appearance suit my site.
The panels used by other forums such as Active Forums is also the exact same sort of thing.
There is a commercial RSS module by Orizont that might be good too... and I have just seen some excellent RSS content available for free on DNNCreative.
I'm going to start a thread now asking how to RSS the whole forum.
Rob