I've just noticed in validating my feed and looking at the feed on my wordpress 2.7.1 blog with ie7, at the end of the post, the strings aaaaaa are added to each post, this text is not included in the actual blog post, and is only on the rss feed.
have a look
http://gravelpitallotments.org.uk/feed, you see what I mean, just looks a bit weird.
I'm running 1.6.6 of simply tags, and is only when I activate simply tags this occurs.
Andrew