Asychronous database replication

I have a project on my plate which will involve potentially hundreds of
PG8 databases in the field which will need to synchronize data with a
central database.  The company is a secular nonprofit which delivers
medical services to underprivileged kids as well as to disaster victims
like those hit by Katrina.  We have six mobile medical units there now
as a matter of fact.

Some of these databases will have 24/7 net connections; some may not
even have telephone access for days so traditional database replication
techniques won't work.  I've not found any third-party software yet
which could help us here so I'm proceeding on the assumption that we're
going to need to build it ourselves.

This sort of database topography is virgin ground for me but I'm
guessing that others here have encountered this challenge before and
will have some tips/advice/war stories to steer us in the right direction.