Worst first post ever.
Casascius has been around since just about the dawn of Bitcoin. AFAIK, there have been exactly 0 accusations of him keeping and using coin privkeys (keeping in mind Goat, one of the most temperamental Bitcoin users of significant public awareness, has had thousands of bitcoins in Cas coins).
I like Casascius and everything but I don't really agree with this assessment. He could have kept all the keys, waiting for the coins to be valuable enough to steal everything. Right now there are tens of millions of dollars worth of his coins; few people have the integrity to walk away from such amounts, and past performance (in this case, Casascius' good form so far) is not an indication of future results. If he kept the keys, he has much more to gain by stealing everything than to lose.
Of course, this threat model is still much better than if he could make the decision to keep keys retroactively.