Best practice for 8.1.5 -> 8.2 migration - with big database?

From what I've read about postgres 8.2, upgrading from any previous
version requires a full database dump and restore.

I am working with largish database (~70GB) that I would like to upgrade to 8.2.
A full database dump currently takes ~3 hrs, I expect a restore
require a similar timeframe if not more.

So, I'm fishing to see of there are alternatives to taking our
production database down for 6+ hours do this upgrade. Can slony or
wal files be of help here?

I realize I can do a pg_dump with the database online, but that dump
will not include data added after the data dump for each table starts,
correct? Is there way to get just the data after a certain point in
time (say after the pg_dump started)?

Also -- due to hardware limitations, we need to install 8.2.x on the
same hardware 8.1.5 is currently running on. Can those two versions
exist side-by-side? If something goes wrong, I want to be able to
rollback to 8.1.5.

thanks in advance,
Mason