check for missing tablespaces?

I am in the middle of helping a customer recover from a situation where 
a tablespace was missing when a machine was rebooted and postgres 
restarted, and I'm wondering if we should not have some sort of check 
for this on startup. Maybe we could check for the existence of the 
PG_VERSION file or something like that? Of course, that wouldn't help if 
the tablespace were subsequently lost, but it doesn't seem like a 
terribly expensive or unreasonable sanity check for startup.

cheers

andrew