Apple has stated that AirDrop will support only the following models:

AirDrop supports the following Mac models:

• MacBook Pro (Late 2008 or newer)
• MacBook Air (Late 2010 or newer)
• MacBook (Late 2008 or newer)
• iMac (Early 2009 or newer)
• Mac mini (Mid 2010 or newer)
• Mac Pro (Early 2009 with AirPort Extreme card, or Mid 2010)

I saw mention of more detail on Twitter, but the forced brevity has provided me only limited information from @GlennF (Glenn Fleishman) (here):

AirDrop will require newer Wi-Fi hardware (newer meaning maybe as old as 2008 or 2009) that allows simul. network/ad hoc.

And from @NeoVanGoth ("Sir Neo Codealot") (here):

Warum #Airdrop aug älteren Macs night geht: "it appears that the WiFi radio module must support simultaneous dual-band for AD to work" #Lion

Can I get a more detailed, and hopefully highly technical explanation about the differences in these network adaptors, as well as what it is specifically that the new ones are doing in order to make AirDrop work properly?