webMethods Advantage is your best resource for identifying the service packs and fixes that are available for any webMethods product including IS and Broker.
While I would suggest that everyone using IS 6.1 should be running SP1, there are certainly circumstances in which an organization might not need the fixes or enhancements included in that service pack or at least not need them enough to justify the effort to regression test their portfolio of integration applications with the new SP.
You can view the release notes for each fix and service pack on Advantage. In addition, you can subscribe to the “Release Notifications” forum which will send you an email any time a new SP or critical update is released.
Advantage has made great improvements in being able to quickly find out what services packs or fixes have been released for a particular product. However, I am still surprised on a regular basis that some fix for a known issue was released and I never knew it. I guess I might not be paying close enough attention, but I also suspect that there is a certain amount of reluctance at WM to widely publicize a defect that may not apply to many (or most) customers.