OK, earlier I wrote that using the Mac's wireless interface en1 simply doesn't working with bridging. I only have anecdotal evidence, but since you were able to verify it, we can take it as a fact.
But let's have a look at the available documentation, which is interesting for itself. Maybe someone else can comment on that?
I had a look at the documentation which was shipped with VirtualBox 4.1.20 (r80170). It says in chapter 6.4 Bridged networking:
To enable bridged networking, all you need to do is to open the
Settings dialog of a virtual machine, go to the “Network” page and
select “Bridged network” in the drop down list for the “Attached to”
field. Finally, select desired host interface from the list at the
bottom of the page, which contains the physical network interfaces of
your systems. On a typical MacBook, for example, this will allow you
to select between “en1: AirPort” (which is the wireless interface) and
“en0: Ethernet”, which represents the interface with a network cable.
OK, this seems to imply, that en1 should be working. But, just the next paragraph says:
Note: Bridging to a wireless interface is done differently from
bridging to a wired interface, because most wireless adapters do not
support promiscuous mode. All traffic has to use the MAC address of
the host’s wireless adapter, and therefore VirtualBox needs to replace
the source MAC address in the Ethernet header of an outgoing packet to
make sure the reply will be sent to the host interface. When
VirtualBox sees an incoming packet with a destination IP address
that belongs to one of the virtual machine adapters it replaces the
destination MAC address in the Ethernet header with the VM adapter’s
MAC address and passes it on. VirtualBox examines ARP and DHCP packets
in order to learn the IP addresses of virtual machines.
OK? I'm not too sure about the capabilities of the airport card, but when I run tcpdump I see the message en1: promiscuous mode enable succeeded in dmesg. (Although I can't see any traffic which isn't mine, or multi- or broadcast. Meh)
Now, the fun part:
Depending on your host operating system, the following limitations
should be kept in mind: On Macintosh hosts, functionality is limited
when using AirPort (the Mac’s wireless net- working) for bridged
networking. Currently, VirtualBox supports only IPv4 over AirPort. For
other protocols such as IPv6 and IPX, you must choose a wired
interface.
I can imagine that Oracle made assumptions about the capabilities. Or, it used to work at some point (back when SUN was still in charge) and doesn't do any more with recent Macs or OSX-versions. I'm wildly guessing there, because it never worked for me.
