What do you call a transformer separated from a wall plug

You're in luck: I just posted a detailed teardown and schematic of the iPhone charger. Internally it's a complex flyback quasi-resonant switching power supply controlled by a L6565.

Building your own switching power supply is probably more complexity and danger than you want to include in a project. I'd recommend going with the wall wart - there's a reason most products use one. If you really want a built-in power supply, I'd recommend getting a pre-built OEM one rather than building your own. And if you really want to build your own, I'd recommend a simple linear power supply instead of a switcher.

In my experience RCA plugs make a very tight connection, and I expect the resistance to be way less than 50 mΩ, probably near 10 mΩ. Even at 2 times 50 mΩ (you have to count both pin and sleeve) a 500 mA current will cause a 25 mW dissipation, which is more than acceptable.

Matt makes a valid point about mixing them up with audio plugs, though. The wall-wart probably has a DC jack like this

Why not use a socket for that? These are almost exclusively used for power supplies, so there's little chance of confusing them for something else. Sockets for them are readily available.