The upgrade to WP 1.5 worked so far - no troubles, BUT:
It seems that there are problems with the 404 Error handling in the wp-blog-header.php file, which is used by wp-rss*.php files.
I couldnt figure out what is causing that problem, so i'll just explain what "symptoms" it is causing:
When I try to view the wp-rss*.php files in a browser, everything looks fine. But web based services like weblogs.com and pingomatic etc. receive just an 404 when trying to get that file.
Same with feed validators like feedvalidator.org.
I don't use "virtual urls" in my WP installation, and this Problem first appeared when upgrading from 1.2.2 to 1.5.
The URL of my WP installation is http://thinktank.net-zine.de, the URL of the RSS2 generator is http://thinktank.net-zine.de/wp-rss2.php.