While 3D printing is way cool, it's not really cost-effective except for single items or very small runs. And it would definitely not lower production costs for mining hardware, because chips, PCBs and cabling are the main factors there.
For personal fun and experiments, 3D printed miner cases would be nice - think about having a personalized case for a USB miner (mining-wise it would still be almost worthless, but much more fun to show around).
If you're looking for a bitcoin-related 3D printing business idea, producing 3D printed items for people who want to pay in BTC might be more feasible, although the demand is probably not high enough at this time.

Onkel Paul