One might say that as a worm farmer you see treasure where others see trash. Cardboard is one treasure that is very plentiful and easy to find but the problem is that in its raw state it is very unmanageable in a worm bin.
For example if you were to wet cardboard boxes and place them into a worm bin they would eventually be consumed and turned into castings but this would take a long time. Also until the boxes broke down they would be in the way of adding new foods, taking up lots of space, and make sorting or separating of the worms difficult.
However when cardboard is first pulverized it makes an excellent bedding and the worms do seem to get some nutrition from it. I have tried raising worms on straight pulped cardboard and though the worms do survive and even multiply a little it seems that they do best when another food source is used and the cardboard is treated as a bedding. I have even seen that the red wiggler reproduction rates can be increased by the worms growing in cardboard. As far as the actual pulping of the cardboard is concerned it can be very labor intensive.
Lately I have been finding pre-shredded cardboard coming from companies that do lots of shipping. They use specialized cardboard shredding machines make a sort of packaging material from old cardboard boxes. The great part about this stuff is that it is already in a fairly use-able form for the worms. Also, the paper fibers are now cut to short and it cannot be recycled so there is no better use for it. This stuff kind of looks like a fish net made from cardboard and the worms love it so keep your eyes peeled for that bit of worm treasure.
If you happen to have access to a hammer mill than you are in good shape. That is what we use to make our bedding with.