The AT&T 3G Microcells are for sale in San Diego, and coincidently, we're down there for the holidays. I went to one of the stores that was selling them, and despite being told we were from out of town, they were happy to sell us one. Registering it online was simple, although it did take about a half an hour for it to complete its setup and become active. During that time, you can't help but wonder if it is going to succeed. It needs to get good GPS reception before it will fire up. But once it did, the coverage at our parents' house was quite good upstairs. It was a bit more spotty downstairs, but once we get home I am fairly confident that the coverage should fill our smaller house. The web page at the AT&T site that manages the microcell allows you to put multiple physical addresses in and pick which one is the active one. You must put an address in, and the device's GPS will verify that it's accurate or it will refuse to work.
Normally you'd install the microcell someplace where you already have WiFi, so the fact that the microcell does 3G data is kind of moot. There are some points of interest, however, that make pondering the trade-off worthwhile. With WiFi turned off, I get a fairly consistent result from speed tests: a little less than 1 MB/sec down and about 60 Kbps up. WiFi here at my parents' house in San Diego gets easily 5 MB down and 2 or 3 up (their cable modem service is amazing - like, 15 MB down and sometimes 5 up). However, with WiFi, you're typically behind a NAT, and if you use 3G, you get a public address - even though you're using the microcell. Of course, apps that require local connectivity, like the iTunes remote app, won't work through the microcell because of that (you'll have to use WiFi).
I've made a number of calls through the microcell now, and they all sound as good as a cell phone can get. When we get back to Santa Clara, the uplink bandwidth will be a bit more constricted than it is here, but given that Vonage works ok, I would think the microcell will work just as well.
Update - 12/27
We got back tonight. Before leaving San Diego, I powered off the microcell and changed the address on the unit to our home address here in Santa Clara. When we got back home, the microcell took about a half an hour to come back up. When it did, we got the activation text message again, just like after the first activation. As I predicted, the coverage is just what I had hoped - 5 bars throughout the house and all around our property. Unfortunately, there is no unlimited minutes offer outside of the test markets, so we're not getting any breaks on our plan minutes. But hopefully that will change when the nationwide rollout starts. I suspect that the various test markets are really about testing the price points for those plans rather than any technical issues.