I did think of it... which is why I went with regular inkjet on paper for the printing.  And the printing is relatively light, and combined with other printed marks I have made on the private key sheets that help me ensure they have been placed with the right pre-printed hologram sticker.

During the decision making process, I asked myself this same question.  I felt that if I used laser printing, that the fused toner could be detectable through ultrasound or that it might reflect some sort of radiation or have magnetic properties that could be detected.  So I completely steered clear.  And if I had done anything engraved, it almost certainly would be detectable in reflections of radiation.

So... I suppose I would be interested if someone finds a way.  I suspect these coins are going to be collected more than circulated, and a major objective I had in producing the coins is simply to seed the concept that Bitcoins can be tangible and can be a backing and they don't have to exist purely in cyberspace.

If someone finds a way to compromise my coins, their utility will be reduced somewhat - people just won't be able to circulate them amongst strangers.  I take comfort in suspecting that's not happening a whole lot, as compared to people giving away the coins as gifts or to people who they know.  (Example: I gave a family member bitcoins for babysitting...they would have done it for free but the circumstances dictated that paying them something was appropriate...and that something was bitcoins.)