high availability with Open Source should be quite common. There are several Projects and papers about this issue.
I wrote my diploma thesis about that *siiggh*, where i compared three Solutions for a HA-framework.
Redundancy in mind, it should be determined first, what's important to keep alive. The core? The DB? The Filesystem?both? How about the TV Cards (keep in mind THE week w/o TV!)?. The easiest way to increase HA is to go for RAID 1, hoping Murphy won't grill both HDs, so you just can keep a second "standby" Core and switch HD, rebuild RAID1.
Things like heartbeat etc could do the trick as well.
The "winner" of the conclusion was http://www.openais.org, an opensource implementation of the Service Availability Forum, as the idea was to keep several independant Software Moduls online.
The company i worked for never implemented anything based on my thesis .... i wonder why ;O)
Havin' this in mind... HA is pure pain!
and a presentation: http://www.linux-ha.org/_cache/HeartbeatTutorials__LWCE08-ha-tutorial.pdf