handling 20,000 connection?

Hi,

I need to set up a system that will be used by some 20,000 users. Most
of the applications will be web based.
I'm wondering if postgress will be able to handle 20,000 connection.
MySQL has a max connection of about 16,000. Does postgres have a max
connection limit? or does the limit depend on the hardware resources only?
I wonder if using a cluster will help (something like oracle database
cluster). I have search the mailing list for clustering, and found some
articles

http://archives.postgresql.org/pgsql-general/2000-12/msg00024.php

but this was quite long time ago, and it seems like they are still in
research.
Has anyone done postgres clustering for a production use?
It would be helpful to have a section explaining how to do this (if it's
possible).

thanks,
- reynard