WordPress 2.7.1 is now available for downlad. This version is a maintenance release and fixes a number of minor issues that were found within WordPress 2.7. You can automatically upgrade from 2.7 to 2.7.1 from within your WordPress 2.7 Admin panel via the Tools/Upgrade menu, or you can download the package and do the upgrade manually.
So while this is just a maintenance release, it is not necessary to update immediately as no major security issues are involved. However, if you like to keep fully up to date with WordPress it is a good idea to do so.
It is easier that ever to update WordPress by using the admin panel feature.