I had trouble when installing the SimplePie Core plugin (v1.1.1) on my WordPress (v3.0.1) based site.
I've used SimplePie quite a bit in non-WordPress sites, and after activating the Core plugin I pasted my standard SimplePie code (minus the "require_once") into theloop.php. Although the SimplePie call basically worked, every time it also threw this error:
Warning: ./cache/7d94a604b2079c5a34e39734359f3146.spc is not writeable in /wordpress/wp-content/plugins/simplepie-core/simplepie.inc on line 1769
The SimplePie documentation and the WordPress Codex don't seem to document the step of manually creating the /cache directory. I think it should be documented because it doesn't seem obvious: on the two sites where I used it, I had to create the cache directory in different places.
On one site it was in the WordPress root:
and on another site it was in the site root:
It seems to depend on whether WordPress is configured to take over the entire site.
(Googling turned up several places that said to create [wordpress]/content/cache, but that didn't work on either site where I tried it.)
Could someone who knows the real story add an Installation step on the WordPress Codex entry for the Core plugin? I've made a parallel request for someone who knows the real story to update the SimplePie site.