Linux & HA


Linux (and *nix) are strong choices for HA.
The inherent stability of the OS adds to the reliability of the enterprise environment that is being made highly available.
The flexibility in the networking and file systems allows for extremely complicated failovers, involving multiple servers/applications.
Ease of scripting also makes life a lot easier (trust me, i have done HA on NT)
Built in IP Aliasing also makes failovers seamless to the outside world. 
HA applications in linux can take advantage of having advanced networking built into the OS, and can thus run with very little loadtime.
Not to mention, for basic HA needs, there are open source applications that do the job a LOT cheaper than the equivalent NT apps.