I recently installed 1.5.
I can pull up my feeds in Firefox and read the xml.
I have subscribed to my wp-rss2.php feed in Thunderbird and it updates with new posts just fine.
I tried to LiveJournal syndicate the same feed (multiple times) and got errors saying the feed was invalid or unavailable.
So I went to two different feed validators, both of which returned a 404 error for the very same feed URL that I am refreshing in my browser and seeing with no problem: