LEGO bricks are made to fairly rigorous standards. I suspect DIY bricks would be closer to existing knock-offs: very much like LEGO bricks, but they don't necessarily fit together "just so" all the time.

They are out of patent protection, which is why there are already clones. However there seems to be some copyright protection still on some bricks http://en.wikipedia.org/wiki/The_Lego_Group#Trademark_and_pa...

But it would be lovely, just printing out new blocks for the kids...