i'm sorry for a gavino-style posting, but..
recently i've found Terracotta -- "open source clustering for Java".
from conceptual side (haven't tried it yet, but they say they support it in
production environments) it's extremely cool -- writing clustered
applications becomes as easy as writing multithreaded.
you simply write application as usual, and then specify in the configuration
files which objects you want to be shared between instances -- and they
become shared, all objects from the root one.
standard Java multithreaded synchronization primitives can be used to
synchronize stuff.
and they say it's handled very efficiently -- only stuff needed to be sent
is sent, etc.
does Lisp have clustering like that? :)
well, i suspect ABCL (working on JVM) might work with Terracotta clustering,
but it might be not as convenient for Lisp as it is for Java..