For the technically inclined: what I did was update Simplepie to the latest development version, and alter a few things in Moonmoon to work with it (several functions changed names, and one now expects seconds instead of minutes, for instance). I also had to edit Simplepie itself so that it prefers the full content instead of the "description" field, which is a small excerpt of the post; the full version of PA is meant to have full posts, as long as the members provide them. If you do want excerpts, you can use Planet Atheism - short posts version instead. :)
Any problems, please let me know (just comment on this post, or email me at the address provided in the PA sidebar).
EDIT: thanks to gsnedders for telling me that Simplepie does have a function to get the entire post; it was just that Moonmoon was attempting to get the description instead. A simple change to Moonmoon (
get_contents()), and I was able to go back to the unaltered