WordPress 1.5.1 has been released today and here are some of the changes.

» Login and feed fixes for IIS
» Faster gettext i18n
» Improved i18n string coverage
» Extended ping support
» Paging on the Manage->Posts page
» URI-safe accent stripping for all UTF-8 characters in the Latin Extended-A Unicode block
» Query string style argument list support for wp_get_links() and wp_get_linksbyname()
» Improved hierarchy listing in wp_list_pages()
» Support for a Status: theme header field that allows themes to be marked as private, publish, or draft
» Improved caching and database query reduction
» Active plugin and theme highlighting
» Plugins can now have multiple option pages
» Pingbacks now work on hosts with fopen off like Dreamhost
» Many bug fixes

I don’t think I will upgrade anytime soon because it took me quite sometime to hack some of the core files to make it fit into my site. Of course, unless there are some secruity loopholes in 1.5 then I will upgrade immediately