I'd say it would depend on whether you can accept the downtime. If it isn't 24x7 critical, I'd time a dump and restore and see how long it would take before doing it for real.
Otherwise, if you aren't going to continue replicating after switching to the new server, you're going to spend a lot of time installing the old version of slony, setting up the replication, waiting for everything to be copied over, switching over to the new master server, then shutting down all the slony stuff.
If you are going to upgrade the old server and continue replicating, you'll probably end up doing all of the above with the 1.2 slony, then upgrading to the 2.0 slony. Might as well dump and restore, then skip straight to configuring slony 2.0.
If you're going to continue replicating between 9.0 and 8.1, then go ahead and use slony to copy everything over and leave it running in slony 1.2 after the switchover (the switchover instructions linked above make the former master a slave of the new master, so replication automatically "reverses").