Thanks to a comrade in RHEL 5 mailing-list! Yeah, it seems RHEL 5.5 ISO’s are now available at RHN.
As of now, there’s no official announcement from Red Hat yet but RHEL 5.5 Release Notes is available at http://www.redhat.com/docs/en-US/Red_Hat_Enterprise_Linux/5.5/html/Release_Notes/
[Update] Press release is now available at http://www.redhat.com/about/news/prarchive/2010/rhel-55.html
[Update 2] Red Hat Enterprise Linux 5.5 GA Announcement is now available via mailing-list.