First, I'd say dump the wireless requirement, at least for the early prototypes. Once you have a prototype that works, and you've picked up some electrical knowledge along the way, you can add-in wireless after-the-fact. Assuming you've designed the firmware well, it should be fairly easiy.
Then, I would say the approach I would recommend would be to target a microcontroller that can easily emulate a HID device.
The cheap and easy approach, and the one I would take, is to buy an arduino leonardo. The leonardo (and the makey makey, for that matter), both use an ATmega32U4, which is a microcontroller with an integrated USB interface.
Since the USB interface is part of the microcontroller, rather then a separate, purpose-specific device, it can be configured to act as a arbitrary HID (human interface device). In fact, there already exists a library for using a ATmega32U4 as a USB keyboard.
Now, lastly, you are basically almost certainly going to have to use a switch-matrix of some sort. Aside from designing your own circuit-board, with an enormous IC (such as a 144 pin TQFP, or similar), you are not going to have enough IO lines to have a dedicated input for every key.
This is fine. Switch matrices are a well-understood practice, and if you're really concerned about button aliasing, you can add a diode for every switch, and make the circuit-board incapable of aliasing.
For the moment, I would suggest you buy an arduino leonardo, and throw together a prototype. I think you're underestimating the mechanical complexity of this build significantly, and having the electronics you need to at least get the system talking to the computer, and acting as a keyboard will let you start poking around at the mechanics.
Best Answer
I did something similar with the original Xbox controller. That used a "proprietary" USB connection. It was USB 1.1 plus a fifth wire that was used as an identifier or something. I just ignored it. The USB just sent HID packets which were easy to interpret.
The PS3 controller seems very similar but looks to use just standard USB communicating in HID (Human Interface Device) format or Bluetooth communicating over HCI (Host Controller Interface) and again sending HID formatted commands. So you can connect the PS3 controller directly over USB to the Arduino Host Shield or connect over Bluetooth using a Bluetooth dongle.
This article gives a detailed overview of both types of connections including links to the various standards and LGPL source code for you to try. Here is another article where the USB section links back to the first article. But the Bluetooth section gives a quick view of the HCI packet format and a step by step on how to pair with it.