I was also hoping for some good splog/feed stealing drama when i first read the subject
I don't have yet seen a plugin for wp that can plug rss feed content in the sidebar, thumblogger can do it, but that's not your pick. There are little services that will parse the rss feed for you and give you a bit javascript code, if you put that code in the sidebar the content of the rss feed will be displayed. But that is javascript.