We're not an ultra-high-volume forum site (yet!), but since we have a server farm anyway, we threw GF onto it:

We have the GF scripts and templates on four Apache servers behind a load-balancing switch and a replicator (which takes care of moving changed templates and modules around). They talk to MySQL on a dedicated database server (eventually, we'll get around to changing that over to a pair of Oracle databases). No mod_perl yet. So far, so good!