[Velocd]

I don't think there are any 5000posts/minute forums out there

While vBulletin may be able to contain tons and tons of members and still function, it can't sufficiently manage the bandwidth that gets passed around, not to mention the server load generated. I've been on huge vBulletin sites that go slow as hell when trying to navigate. (obviously other factors go into consideration here)

Jive and other software built on Java or more stable platforms than PHP&MySQL have a better edge at the enterprise market.

For a large business, stability is the most priceless asset. Even somewhat negligible downtime would likely ensure loss of clients.

vBulletin stands above in features, but as mentioned (I think), abundant features aren't important for enterprise solutions.