Our HiFiBerry project is still ongoing. The PCB design is almost finalized and we expect the device to be ready in about 4-6 weeks. With HiFiBerry you will get an inexpensive and high-quality sound card for the Raspberry Pi. However, there is one thing it cannot provide: high-resolution sound, that means sampling frequencies above 48kHz are not supported. There are a lot of Pros and Cons for or against higher samples rates than 48kHz. At least when it comes to post processing like equalizing or digital crossovers, higher samples are a good idea.
You could also add an external USB sound card. But we are looking for a real DIY solution ;-) The 2nd revision of the Raspberry Pi provides access to the I2S pins of the processor. You can add an I2S capable ADC or DAC on these pins.
Unfortunately, the Linux kernel of the standard Raspberry Pi does not support devices connected to the I2S pins. Therefore you need to compile your own kernel. Check out the “Noise is good” blog for more information. Hmm, looks like an interesting project for another Raspberry add-on board. We will have a look into this.