"Less is more"
I2s was invented as a standard for transferring digital data between CD spinnner mechanism and DAC chip(s) inside CD players. Thus all audio DAC chips (that I know of) have I2S input pins. "Wires" of I2S transport can be directly connected to DAC chip - there is no need for other intermediate digital signal conversions (like conversions to and from AES or SPDIF) which is, of course - better. But, as mentioned, I2S was designed as standard for internal digital signal transport, not external. AES and SPDIF were invented for external. Thus I2S signals are susceptible to various interferences and, in general, I2S external cables should be shielded and short.
Note that there is no guarantee of better sound with I2S transport. In theory it is the easiest, most elegant way of feeding digital data from transport (computer, digital player...) to DAC chip, but in practice it depends on the implementation (as all other things in audio) of digital data conversion to I2S format and of I2S signal transfer from I2S convertor to DAC chip.
In general, we are talking here of USB to I2S converters.
Note that on most DACs with USB inputs - the USB inputs are in fact "internal USB to I2S converters". Internal in a sense that the converter is physically located inside DAC and once USB data from it is transformed into I2S signal - this I2S signal has to travel only a few cms to DAC chip which is, very likely, on the same board. Ideal place to put USB to I2S converter/input, no? Yes, but we know that up till recently, almost as a rule DAC USB inputs were blatantly inferior to SPDIF inputs on the same DAC. This demonstrates the importance of quality implementation of USB to I2S converters (converter power supply, clocks, clocks power supply, converter chipset, converter firmware, type of USB transport, converter software drivers...). Fortunately - things are moving for the better and one can foresee USB domination in the future...
It is almost the same with external USB to I2S converters. That is - with the additional problem of transferring impeccable I2S signal through I2S output connector (ethernet socket in most cases), through cable (ethernet cable in most cases), DAC's I2S input connector (again ethernet socket) and wires or board traces to DAC chip. See why, all other things equal, internal USB inputs have advantage over their external counterparts?
So ohhgourami - to connect your computer to your DAC through I2S you would need some kind of external USB to I2S converter.
Or get a DAC with decent USB input.
Or even better - get DAC with decent USB input and I2S input. So you can enjoy your music now and try latest and greatest external USB to I2s converters in the future. Those current are either mediocre or shamelessly expensive.
BTW, A-GD's latest USB-32 implementation of internal USB to I2S is quite good. I am yet undecided, but NFB-11.32 USB input is in whereabouts of audiophilleo2 connected directly to NFB-11.32 BNC SPDIF input.
Edited by FauDrei - 9/22/12 at 11:40am