Yes for RHEL. (Since Feb 22, 2012)
Oracle Offers Free Trial of Innovative Ksplice Zero-Downtime Updates to Red Hat Customers
To continue after the 30 day trial, you need to purchase the
Oracle Linux Premier Support package.
Pricing starts at $1,400 for systems with two physical CPUs and $2,300 for systems with more than two physical CPUs. You can configure your systems to automatically install updates as they become available.
It had been offered for both RHEL and CentOS (as well as other operating systems) prior to July, 2011 when Oracle acquired Ksplice. Prior customers are still supported.
Oracle buys Ksplice
The Ksplice code was created and distributed under the GPL v2 and other open source licenses. After Oracle acquired Ksplice and limited availability to Oracle Linux, Ubuntu Desktop, and Fedora, there was discussion about maintaining a CentOS fork of Ksplice:
Ksplice and CentOS
It was also argued that Ksplice was violating the GPL:
Ksplice currently violates the GPL