Back from vacation today and my wife did a software upgrade to iOS 5.1 on her AT&T Wireless service iPhone 4S. Sure enough her indicator now reads "4G" (upper left hand corner) and she's getting some pretty nice bandwidth. Here's a screen shot after running the speedtest.net app.
As a comparison, here's the same test run on my AT&T 3GS iphone.
I'm also updated to iOS 5.1 but no "4G" for me on my older phone.
Is it really "4G" on her phone? Well - no. It's running a 3G service called HSPA+ (sort of 3G on steroids - up to 56 Mbit/s downstream and 22 Mbit/s upstream) which AT&T is using to get over the bandwidth hump until LTE rolls out. I've written about 4G services fairly extensively - follow this link to read my older posts.
Regardless, that's some sweet bandwidth. If you've got an iPhone 4GS running on the AT&T network, be sure you're up to date on upgrades. As for me and my almost 3 year old 3GS - I'm stuck in the "3G" world until I upgrade my phone. The 3GS does not have a HSPA+ radio in it.
How can you check which iOS version you are running on your iPhone? On your phone select "Settings", "General" and then "Software Update". You should see a screen like the screen shot from my 3GS below.
If you are not up to date follow the instructions on the Apple website.