[Zope] replication, clustering, scalability

Eventually you get more traffic than 1 server can handle. When you have code being replicated to multiple machines, all of which talk to a single SQL back end, you get a certain kind of flexibility... Replicating that code can be tricky - it's a pain to move around Java classes, on the other hand ColdFusion files copy quite nicely. But as code and data get intermingled, this gets trickier. In my old days of running a Frontier-based Mac web server, I ran different parts of my site on different servers. That can work, at least for a while. But if feels ugly, like I'll regret it later. I rather prefer running a farm of identical machines. How are others handling this?