I also have an Alu iMac (2Ghz), which has the a/b/g/n wireless adapter (shown in the Network Utility).
Now I put the MacBook Pro in Target Disk Mode, connected it to the iMac (FW800 to FW400 cable), and guess what.... now it only shows an a/b/g wireless adapter.
That's really strange, as only the MBP's HDD is used as a startup volume, and not any other component. SO it seems to be a driver issue.
But putting the iMac into Target Disk Mode helps nothing, as the MBP reboots time after time, only showing a blank grey screen, then turning the display off, and then playing the notorious chime. And that in a loop.
I try again with my MacBook, which has a working a/b/g/n wireless adapter.
The MacBook as a startup volume shows me, that I have an a/b/g/n adapter.
The MacBook has 10.5.6, the MBP has 10.5.5 (downgraded from 10.5.6 as it removed the trackpad feature and was suspect of really abysmal battery drainage) and the iMac has 10.5.5.
So the culprit might be 10.5.5. I'll see later.
PS: How to roll back to 10.5.5 from 10.5.6, without reinstalling? Will the 10.5.5 Combo Updater from Apple's Support site do?