Well, there are two networks - 1 at the regular 2.4GHz and 1 at 5GHz
B/G/N (2.4GHz): Where most of your traffic is going to go. (DS/PSP/PS3/360/etc)
A/N (5GHz): Where some specialized traffic will go (PCs with N adapters, iPad, etc)
The two frequencies don't bug out each other, but if you, say, put an .n device on the 2.4GHz band along with .g devices, the .n is going to be throttled down whenever the .g device is in use. (This isn't a problem at 5GHz since few devices support it, and there's no .g at that band) I know the iPhone 4 supports .n but I'm not sure if it can work at 5GHz, I'll give that a try tonight (just got my phone today.)
You can setup a second network on each frequency (typically called a "guest network"), which allows you to specify a different encryption and settings, but again, I think if .g and .n coexist it could be throttled? It's more useful to have as a real guest network where you can specify different settings (as I said, you can have a .b/.g WEP network for your DS that you can disable when you don't need it, but keep everything more secure on a .g/.n WPA2 network, like your PC, 360, Wii, and so on).