Mark,

I would recommend reading the pgsql performance link I've included  
below.  You can see that I too was flirting with pgmemcache.  The two
issues that prevented me from using it were

1) The race condition that exists when the memcache and postgres data  
are out of sync and
2) The integrity of the data in the cache

The thought is that a new PG feature, an ON COMMIT trigger (there is  
some debate, at least in my mind, whether BEFORE and/or AFTER COMMIT  
would be useful), or ON PREPARED TRANSACTION, would solve both these  
problems given PG's MVCC model.  The race condition would be greatly  
diminished since PG and memcache would be updated nearly at the same  
time.  Also, if the transaction failed the cache wouldn't be updated.

Unfortunately, this doesn't exist, so to me pgmemcache has limited  
use unless you believe all your transactions will always succeed and  
you don't care about race conditions.

Hopefully that helps you in your decision.

http://groups-beta.google.com/group/pgsql.performance/browse_frm/ 
thread/f345107f382a7aee?tvc=1&q=pgmemcache&hl=en

Christian