For the uninitiated, here's MSFT's description of ClickOnce that you can say with your mouth full of food:
ClickOnce, part of version 2.0 of the Microsoft® .NET Framework, allows you to deploy Windows-based rich client apps to a desktop by placing the application files on a Web or file server accessible to the client and providing the user with a link. (source)
But when I stepped back and looked at ClickOnce from 10,000 feet, I began to see some opportunities for connecting the pipe (RSS) to rich content on the pipe. To the extent that some of that rich content may require specialized client software to present it, providers will need to figure out how to get that client software onto their reader's machines in a safe and scalable way.
One reason that ClickOnce works as a packaging and delivery mechanism for individual content is that it operates in a secure sandbox provided by .NET code access security (which you'd know if you'd read the article referenced above). This is a level of security that administrators like.
Rate-limiting factors are, of course, the presence of the .NET framework on your reader's computer, and the fact that ClickOnce won't really come into its own until Vista ships - at that time, ClickOnce-packaged code will be able to (according to MSFT) "trickle-feed" update of files in a way that is less obtrusive to the user. That way if you deliver some executable via ClickOnce that is used to enhance your RSS content, you can update it in the background independently.
Right now, ClickOnce looks good for delivering richer forms-based panels and whatnot via browsers. Who knows what it'll be good for later in 2006. But whatever it is, it's a nice lightweight, secure packaging technology that might yield some good synergies with content you want to deliver via RSS.