I've looked into this sort of thing too, I considered using Network Block Device (
http://nbd.sourceforge.net/ ) which is essentially having a raid array split between disparate computers.
So you could have two servers, each with a set of drives set up in raid 5 via hardware, then you could use the NBD driver to mirror those so they were always in sync, all writes would end up on both sets of drives.
Anyway, I tried it, and couldnt get it working, your mileage may vary. I think I'd prefer to wait and see what the Zimbra crew come up with. Hacking your way under zimbra to make it fault tolerant is one thing, but having zimbra do it itself has to be a better way.