Err.. @DougS: actually, it is standardized, part of the IMS 3GPP specs (as the article helpfully notes). And it works really, really, well. I've been happily using it with T-Mobile USA for at least 5 years (the first time in the UK was in late 2010, when I happily connected to a BT hotspot in the Highlands and got cellphone coverage where no-one else could, and without roaming (to call the USA, naturally). Network performance is an issue, and it will refuse to enable itself if the network connection isn't satisfactory, but it works with a Three WiFi dongle, but not with a satellite service.
On American Airlines aircraft in flight, it's used to provide SMS access (for free), although not voice (thank god). MMS doesn't work unless you've bought the generic internet access service (from Gogo Inflight), which is totally acceptable once you've figured out your phones predilection for using MMS instead of simple SMS.
It's supported on T-Mobile's higher end phones, including Android, iOS and Windows. Right out of the box, too: you just select WiFi preferred, Cellular preferred, or never use Cellular. The only snag is that if you are used to wandering around while on the phone, you have to remember not to walk too far away from the hotspot.
Of course, these days it's less essential, since T-Mobile has essentially given up raping for international calling, data services and SMS in a reasonable number of countries (including the UK); (sure, the free data rate is capped at 128Kbps, but it means that e.g. Google Maps "just works", to the extent that it just works anywhere, when I walk out of an airport in Europe, Israel, etc.