Complicated question...

802.11n (b/g compatible) only operates on the 2.4GHz frequency (since b/g only operates on it), so no you are not getting 5GHz speeds. You're getting ~170 Mbps maximum.

Assuming you don't have a recently released AEBS, it's perfectly possible to create a b/g access point and have everything run smoothly. In fact, I have a Linksys WRT54GS acting as a b/g WAP with my TC running in 5GHz n only and everything connects just fine. It's even easier to do if you have one of the recently released Extremes since those are dual band capable without any additional hardware.

Lastly, from what I understand, if a b/g device connects, it only slows things down when the connection is active (ie, you're downloading something to your iPhone), and even then it doesn't slow things down completely to 54 Mbps on N enabled devices. But don't take my word since I haven't researched this extensively.

 

1. Since it's in a compatible mode, it's running at 2.4 GHz. If your iPhone is able to connect to your network, then it means it's at 2.4GHz.

2. A lot of people are doing that. And that's the whole big thing with the new AEBS just recently updated. It distributes dual band radio modes, Wireless N only at 5GHz and B/G compatible at 2.4GHz.

3. Yes. It will slow down to G speed.

 

No, it will not slow down to G speeds. I was connected to my b/g/n mixed network with my MB (which is N) and simultaneously with my PC desktop (which is G), both downloading files from the Internet. The transmit rate on my MB, while being less than the usual 170 Mbps when my desktop is off, was actually at ~100 Mbps, not 54 Mbps.