said by birdy :
That 2nd antenna isn't just sitting there to look pretty. The rev I is most likely a dual band N router. The big question is whether its single or dual radio. If single, then only one frequency available (2.4 or 5GHz). If dual radio then 2 SSID's available with one being 2.4GHz channel and other being 5GHz channel.
I don't think your explanation between dual and single radio is technically accurate. I have a Netgear WNDR3300, which is of the single radio variety. It offers separate and simultaneous SSID's on each frequency (2.5 and 5 Ghz), but limits N to EITHER of the two..
ie - You can run B/G on 2.4 Ghz with SSID "Network_X", while also running N on 5 Ghz with SSID "Network_Y", but you can't run N on both of those networks simultaneously - its either/or only in the case of Wireless N. Depending upon the devices you're connecting (capable of 5 Ghz?) and the network topology (most importantly distance and interference), many including myself would be just fine with single radio technology. I get 40 mhz (wide) 5Ghz 270 mbps connections while also supporting my older B/G devices on 2.4 Ghz.
Edit - What's also nice about using a single radio dual band N (which tend to be the cheaper of the dual band N's) - is that when combined with the Rev F Actiontec router, you can also get slightly quicker 2.4 Ghz connections through the Actiontec's own draft-N ability (on its own SSID - with a connection rate of ~65 mbps. Now you're running 3 separate wireless networks, with the only limitation being the speed of the wireless N channel on the 2.4 Ghz band.