I called AT&T as soon as I saw that iOS 4.3 had gone live. When I called, they had no information about setting it up and didn't seem to be ready for any influx of setup requests. But, eventually I was handed off to a technician and that person was able to find out the setup information.
So over the phone, they were able to convert me from my legacy unlimited plan to the monthly 4 gigabyte plan with hotspot. I received a text message saying the data plan had changed, and the hotspot button in the network section of the general settings pane allowed me to access the hotspot settings. I turned it on, changed the password, and saw that it worked fine with my MacBook Pro. I'm using it right now.
When I plugged the syncing cable into my MacBook Pro's USB port, I got a notification of a new network connection. For whatever reason, I had to restart the Mac System Preferences application to get the "iPhone USB" connection in the Network settings panel to connect. I will delete this connection, as I wouldn't want to accidentally spend data when WiFi is available.
The CNET Online Speed Test gave me 1288 kbps which is about what you'd expect for mid-day in Cambridge on AT&T.
A Facetime chat over the Internet to my iPod Touch from my MacBook Pro was surprisingly good. My wife in Nashua only looked a little pixelated and the audio was as good as it ever is.
I'm looking forward to dropping my iPad service, which will save me $15 a month off my old $30+$30 plans, and also allow my kids to use their iPod Touch in the car, and occasional use of my MacBook. I've never come close to using 2GB on either my phone or my iPad, so there is no real downside in terms of cost.
So a great service to finally have on iPhone.