A fresh Haiku installation does not appear to automatically acquire DHCP information, such as when running inside VirtualBox. As a workaround, I am manually executing:
$ ifconfig /dev/net/pcnet/0 down
$ ifconfig /dev/net/pcnet/0 up
Before continuing configuration. Unfortunately, this worked exactly once–When I reinstalled Haiku OS, ifconfig down/up failed to trigger DHCP lease acquisition. Worse, when I tinkered some more with ifconfig, I managed to trigger a kernel panic. Quite the hilarious first time experience with Haiku nightly.
Update
DHCP seems to work better with the older Am79C970A virtual NIC than the newer Am79C973 NIC. So I suppose it’s just a matter of Haiku improving its driver support for Am79C973 NICs. For some reason, Haiku support for this device has worsened between alpha 4 and nightly.