IBM has had HACMP for years on AIX, the Italian Unirel Sistemi has
been selling the EasyLife HA software for Solaris on SparcStorage
Arrays or Clariion diskarrays since at least end-1994... I don't
know about HP or others, if SGI comes out on 6.4 with HA well...
Since a HA solution may be fully implemented in user space (each
box has a "heartbeat" daemon talking over a RS-232 line, when 1
detects the "death" of the 2nd it first goes on Ethernet to check
if the serial line hasn't failed, then if needed looks into its HA
database to see which actions have been configured, just does them)
my guess is that as soon as anybody has a bit more spare time than
I have, a bit of interest in this issue - and of course at least 2
machines :) well a HA solution for Linux can come out pretty easily.
--alessandro