Ksplice [1] lets you do that. For a while they had very limited support for it, but I just checked and they seem to support Ubuntu on the desktop as well [2]. Once this comes to the server edition and/or is bundled by Canonical, using Ubuntu would become much smoother.

[1] http://www.ksplice.com/

[2] http://www.ksplice.com/uptrack/download-ubuntu