[Dovecot] High availability strategies?

[Ed W]

Anyone here trying any "high availability" strategies for mail servers?  
(Ralph what do you do?)

I am reading up a little on DRBD + OCFS for a master/master setup but 
this doesn't seem to address how to avoid having a single master proxy 
process (assuming that we are running on rented equip and don't have 
ability to shift IP addresses around servers).  It appears to be 
possible to play games with DNS and short expiries, but this causes 
extra DNS traffic for the general case when we are up and running?

We had some bad ram in a server recently and it's the worst possible 
situation.  Server locking up, downtime, but it's not easy to repro and 
fix...  (only happens every few days. Memcheck found nothing...)  How 
could we redesign our setup to be resilient against this?  (How does 
Google or Yahoo do it?)

For reference I am using vservers to host dovecot and this is working 
very nicely and makes it real simple to chuck the config onto another 
host and test things (upgrades, etc).  Highly recommended.

Any suggestions (from people who have tried stuff)?

Cheers

Ed W