In first time, sorry for my bad english...
I have a account in Yahoo Webhosting... upgrade to initial WP 2.3.3 to WP 2.6.2. Use a WordPress Automatic Update plugin. Next use a fantastic theme called AGREGADO. Modify default ugly permalinks to better option (year, months, post) and activate Yahoo Customizable Permalinks plugin. In the test all work fine...but (ever had a but) rss feed show XML parser error (this show only in FF3, in IE7 only the tab show error).
Read this theme in:
Next check the next archives:
7. functions.php (in the theme in use)
In this archives fix the issues.
On the brink of madness...:-) check the Yahoo Customizable Permalinks plugin, and discover a blank space after ?>. This file no have assigned rewrite permission...and FTP tools no have utility. Need login in administrative Yahoo interface, next in File Manager, search the file under plugin folder and delete the extra space after ?>...next VOILA...the RSS Feed work fine.
This mode, active nice permalinks and have feed rss OK.
I hope that this experience is very useful.