So here's a quick speed comparison.
I couldn't get it to launch/register properly in a few tries while I had WiFi disabled (either due to their server issues, crappy AT&T signal in my basement, or a combination of the two.)
But I did get it to launch/register properly while connected to my home WiFi. So here's a speed test comparison. First, my network layout, yes, it's convoluted:
Non-Tethered connection:
iMac -> Gigabit Ethernet Cable -> AirPort Extreme -n base station (Gigabit Ethernet) in "Extend my wireless network" mode -> 802.11n wireless connection -> AirPort Extreme -n base station in "router" mode -> Zoom cable modem -> Comcast
Tethered connection:
iMac running Tether.app -> USB cable -> iPhone 4S running Tether app -> 802.11n wireless connection -> AirPort Extreme -n base station in "router" mode (yes, I confirmed that the iPhone was connecting directly to the "main" base station, not the one the iMac is physically wired to.) -> Zoom cable modem -> Comcast
Speed results, via the Comcast speed test site:
Tethered: 10 Mb/s down, 5 Mb/s up
Untethered (aka: iMac direct): 15 Mb/s down, 5 Mb/s up.
This pairs well with the on-phone testing I've done. From where my phone sits when plugged in to my iMac, I get about 10 Mb/s. If I move closer to the main base station, I get up to 20 Mb/s. (But I can't readily move the iMac. I'll test later with a MacBook Pro.)