Bridging ethernet over Wi-Fi?

[Joshua Judson Rosen]

I've noticed that VirtualBox and some other VM packages include
functionality to bridge in-VM network interfaces onto the external
network through a network interface on the host machine. I get how this works
for ethernet, but it seems like they make it work for Wi-Fi, too;
given that Wi-Fi frames don't actually carry enough information to
to do bridging at the station (client) end, how the heck does this
actually work?

The only technique with which I'm really familiar, for bridging ethernet
through an 802.11(b|a|g|n) client interface, involves running the
radio in a non-standard 4-address mode, which requires support
on the AP at the other end of the wireless link; I'm fairly certain
that's not what the VM systems are doing, because they appear to
work with bog standard APs. So what _are_ they doing? Creating
a second (hidden?) interface on VM host with identical MAC address
to the interface inside the VM, and mirroring traffic between them?
Faking it by sniffing and relaying packets at layer 3? Something else?
Maybe something about this actually did get into a companion IEEE
standard that I'm not familiar with?

-- 
"Don't be afraid to ask (λf.((λx.xx) (λr.f(rr))))."