After upgrading to Mac OS X 10.5.4, Safari stops updating the bookmarked RSS feeds. The 10.5.4 update includes Safari 3.1.2 for Leopard.
Previously, to fix the problem I deleted the “Syndication” folder inside my “Library” folder.
I deleted the RSS bookmarks and adding them again, but it still doesn’t fix the problem.
I have not tested this with Safari 3.1.2 on Windows and on Tiger.