The forward looking folks at FeedBurner are tackling the rather tricky legal aspects of splicing feeds together. This will be increasingly important as people try to integrate advertising with webfeeds. Note I'm ignorant/neutral on whether this is a good idea. The key nugget is that if you are seriously publishing using webfeeds, you better start marking your feeds and items with machine readable licenses pronto.
One of those issues I'm glad someone is thinking about, just as long as it ain't me.