Migration to 1.5 via the fine instructions at tamba2 was very smooth. I'm using the "default" Heilemann/Kubrick theme completely unmodified, and 1.5 completely unmodified.
When accessing an unmodified wp-rss2.php link:
Firefox 1.0 says "An external application must be launched. ..."
So, we try FeedDemon, which responds with "Failed to connect..."
Please give me some hints on how to diagnose this problem. FWIW, the site is only a very rough beginning: http://eleaston.com/bob/wp/