With the release of Ubuntu 10.10 Maverick Meerkat, Ksplice Uptrack - the rebootless update capability - will yet again be offered free of charge for Ubuntu Desktop 10.10.
In case you don't know, Ksplice Uptrack lets you apply Kernel updates without having to reboot. While you have to pay a monthly fee to use it for Debian, Red Hat, CentOS or Ubuntu Server, Ksplice Uptrack is free to use for Ubuntu Desktop (from 9.04 to 10.10) and Fedora 13.
Ubuntu .deb downloads and installation instructions @ http://www.ksplice.com/uptrack/download-ubuntu