I discovered the same thing with my fresh installation of 1.5
However, it worked when I added a second post.
First I just edited the default “Hello World” post, adding a <!–more–>, but WP ignored it.
Then I created a totally new post, where I put a <!–more–> into it, and suddenly it worked on both posts.
Don’t know why it does not work on the default post, but first work when new posts are added and updated….strange
It does not work without the “feed:” either
I have the same problem, using latest stable release at:
http://www.sandblogs.com/wp-rss2.php
The feed validator gives a 404 error, and my FeedReader is not able to download any either, but the link work if I put it directly into my FireFox browser