Yeah, they wouldn't limit USB-C to just the MacBook. It'll be in the MacBook Pro in the fall and definitely the iMac in time. And if it's ever updated, the Mac Pro will get a couple too, and hopefully keep the other ports too, but who knows.
The real question is how many other ports will they keep with it. If we're lucky, they won't go all "one port to rule them all!" on the Pro machines. I can see the iMac losing a bunch of ports and just retaining a few C ports, but the Pro machines should keep at least a few other. Right? Well, who knows. From Apple's thinking, USB-C at its best can support every single port they already have with the right adapter. But from the consumer standpoint, no one wants to have to carry every single adapter with them just in case. It's a pickle really. I fear their endgame is definitely only having USB-C ports on their machines with a bunch of adapters that, yes, they will include in the box. At least a standard USB adapter. The others will be extra. And a third party will probably release a USB-C to EVERYTHING POSSIBLE adapter at some point if it doesn't already exist.