How important are transports?

Probably depends on the specific innards of the DAC you are using. Per a musical fidelity dac review in stereophile, that company's designer (Antony Michalson or something like that)says they reclock the bit stream in the DAC, and so which particular transport or cd player with digital output you use doesn't make diddly for difference.
A DAC which does not reclock might be much more sensitive to variations in timing from a less than perfect signal source.
There are a lot of great used cd players with digital outputs, my suspicion is that a great cd player probably does not have poor bitstream timing and tends to be priced a lot more competitively than a transport. Other thing is it is handy to be able to swap back and forth between the cd player output and the dac output when evaluating your system or when your dac is on the fritz.

I wonder if the difference in sounds with different transports is sometimes related to how the particular transport is polluting the incoming 120v line? I do get a better sound from my system with a JPS labs digital cable connected to the cd player I'm using as a transport (Arcam alpha 10 cd, MF Tri-Vista 21).
Maybe not, but if somebody wanted to explore the notion, there is some info on audio asylum about the JPS labs being just a basic cable with a simple r/c network in one of the plug casings, and I believe you can buy it from the original manufacturer for less than $50.00 US.
Motors and digital devices are inheritantly nasty polluters.

Steve N - I appreciate your perspective, but I'm guessing that you don't have your DAC, personal computer, and house fan all plugged in to the same power strip. My only point being that when evaluating audio components that are bad about creating electrical hash on the line (Transports), audible differences may be partly due to power quality differences seen by the rest of your system.