There are actually many different devices to stream audio from your computer to a DAC. Also a few, like the Airport Express, that also include a DAC or are a DAC and can stream an analog audio signal to your preamp. The DAC in the AE is mediocre, but it can be very capable at streaming a digital signal via Toslink (not considered the best way to go as far as digital transmission though). I use a Waveterminal U24 to stream the digital signal, via USB to Waveterminal to DAC via S/PDIF. I cannot tell the difference between my excellent transport and my computer streaming the same material to the same DAC. Which is to say this method works very well indeed for me. There are similar products made by M-Audio and other companies, many of whom are in the Pro-Audio world as opposed to Audiophile manufacturers. Among the latter are only a few marketing USB and Firewire cabable DACS. Wavelength would be one.
Agree on the convenience factor many have mentioned. Your entire library in high-rez lossless format, right at your fingertips. Outstanding!