We recently signed up for Ksplice, a service offering live Linux kernel updates. (Yes, live means while the kernel is running.)
Now, the kinds of updates we are talking about are patches to running code intended to apply critical security fixes. Ksplice won't let you upgrade from 2.6.34 to 2.6.35. Nevertheless, for mission critical servers (especially tough ones like database servers, routers and the like), the prospect of not having to reboot to install security updates is a huge win.
Ksplice is a young company but has racked up an impressive list of clients. I've found their solution easy to use. Billing and support is straightforward, and they're very friendly people. We've had no problems applying the updates. I would recommend Ksplice to anyone looking to keep their production GNU/Linux systems up to date.