The catch is Apple's supply chain management just got a ton simpler as they don't have to maintain separate SKUs by carrier for each iPad variant.
For all 3 major carriers, you just get your phone and in a separate envelope, you get your SIM card.
In reality, there is the technical means to support this on ANY carrier. You just set the IMSI or ESN in software on the microprocessor and then hit an API at the carrier telling them to pair X device with Y unique ID. The difference now is that no one before Apple has had the weight to get the carriers to go along with this, since it encourages modularity and discourages device and carrier lock in.
In US you just get a phone with everything pre installed and sim tool :). So this thing does make it easier for Apple and a bit more convenient for users
I'm in the US and I've almost always bought cheap unlocked phones and SIM cards, but until recently I've had to hunt around to get the right variant of a phone for my preferred provider. Eg. T-Mobile uses 1700/1900/2100Mhz, AT&T uses 700/850/1700/2300.
http://en.wikipedia.org/wiki/Cellular_frequencies#United_Sta...
The latest iPhones and some other phones now have radios that cover all frequencies, but those radios are expensive and typically only come on phones that cost more than $700.
The only toy I remember not having a separate SIM card was Amazon Kindle (the e-ink reader), and that's because they try to hide that the SIM is even there (and it's kind of weird).