As near as I can tell, the SimpleBlog plugin is regenerating <id> tags in the atom feed for all entries whenever a new entry is added. However, if I read this correctly, <id>s are supposed to remain constant for all time for all items. I am also having an issue with SimpleBlog feeds in Google Reader: whenever I add a new entry, all entries reappear. I'm pretty sure the two issues are related--i.e., because the <id>s of all items change, Google Reader thinks the old entries are actually new entries and reshows them.
Is there any chance the SimpleBlog plugin could be updated to maintain consant <id> tags--perhaps using one of the techniques discussed in the link above?