The difference is that this device can switch to wi-fi for voice calls. Not just data. Voice...

The technology behind this is called UMA. Tmobile acts as a gateway between your phone and the GSM network (that is, the phone gets an IP from your home router, then talks to a T-Mobile server, which routes the call). That's why you have to pay them the $10 per month.
If you have ever traveled with a BlackBerry, you know how expensive it is to call home from an hotel or a conference. Ten bucks go in a few minutes... Now, in all those places you have wi-fi. With $10/month, all your calls are included. You just have to sign-up to the hotspot where you are.
I am ready to bet that all CFOs on the planet are preparing to force their employees to take the new device and have them use it with wi-fi, at least at home and in the office, where the majority of the calls happen... It will be a huge saving.
Although it came in quietly, this is a big turning point in the market. Data is now used for voice. It is the start of a new era. Data will be ubiquitous. Blending voice as a data element makes data king. A number of very interesting applications will emerge out of this.

Just based on my gut feeling, I like UMA better... Femtocell gives me the impression I am paying my carrier to improve their network coverage, because it sucks in my house. They should have a better cell tower ;-) And I am not getting any advantage in a hotel, bar or even in the office. I can use my current phone, which is nice, but I am planning to change my phone every 18 months like everyone else. And the next device I get is definitely going to be data capable and have wi-fi, so... Femtocell seems like a stop-gap solution and, therefore, it might not last very long.
In any case, the future of mobile telephony looks very interesting. Wow, it is all changing so fast...