I want to create an IP loopback on my Windows XP Laptop such that...

 IP Packets arriving in on the Wi-Fi interface are reflected back out
on the same interface.

If it is not possible to configure the laptop to do this, is there
hardware out there that could be connected to the laptop to do the
same thing?

(The purpose is to load up a wireless LAN with VoIP Traffic.. VoIP
source will be a  traffic generator on the wired side o0f the access
point which will make calls back to itself via the laptop in the
wireless LAN)

Hi brendank

You mentioned, VOIP traffic generator would make calls to itself. This
means that the VOIP caller and receiver are located on the same
device/machine with same IP address(es).

Is it possible for you to configure traffic generator to send the IP
packets addressed to itself (since caller and receiver have same IP)
towards the XP laptop on wireless LAN?

If yes, then you just need to enable IP routing in XP. To do this
change the following registry entry.

HKEY_LOCAL_MACHINE \SYSTEM\CurrentControlSet\Services\Tcpip\Parameters 
subkey : "IPEnableRouter" entry to '1', which by default is set to "0".

(http://www.windowsnetworking.com/articles_tutorials/w2kprout.html)

Then reboot the laptop. This would configure XP laptop as a router.
Since IP packets arriving from voip traffic generator are addressed to
generator itself, XP would reflect back those.

Please reply if this works or not for you.