I can confirm the change in WP 3.0, and would say it is a bug. This has worked perfectly in 2.9.2 and previous versions.
This is the issue:
Most of our themes use index.php as the homepage in the themes, and often uses query_posts to fetch posts:
<?php $paged = (get_query_var('paged')) ? get_query_var('paged') : 1; query_posts("paged=$paged"); ?>
This works perfectly as homepage, but if you set a static home page in Settings > Reading and set a page as your posts page, then that posts page will not work properly.
Here is an example using TwentyTen with a modified index.php:
Homepage (About): http://cl.ly/ca801b7ac28403d1f3b2
Posts Page (Blog): http://cl.ly/d3cca61eaa5ca2335acf
The solution to this "feature" in WP 3.0 is to set the category parameter in the query like this:
<?php $paged = (get_query_var('paged')) ? get_query_var('paged') : 1; query_posts("cat=-0&paged=$paged"); ?>
Now it works again! http://cl.ly/757da36f56113ed03608
It would be best if this was fixed in the WP 3.0.1 so we don't have to modify a perfectly working query_posts in all themes.