Gotta be a dead cert, surely?
There's enough Chinese coins on Ebay 'inspired' by established designs, though none of them try to pass themselves off as genuine.
It would only take a bit more precision to have a try at making a funded replica but there's probably not enough demand yet.
What if someone seriously invests some time in creating a fake casascius coin that looks and feels exactly the same but dows not have a private key?
And then sells them for 1.3 BTC each or so, claiming it to be the original coin, rare collector's item.
We haven't yet seen this happening, but this might be the same fraud as it happens with fiat money - counterfeit dollar bills.
There is one known case of a Casascius counterfeit, it was listed both on ebay and here on the market place:
https://bitcointalksearch.org/topic/are-there-fake-casascius-coins-yet-437465
http://www.ebay.com/itm/Funded-Casascius-25-BTC-Physical-Bitcoin-2011-V2-/281254829549?pt=US_World_Coins&hash=item417c181ded
The case is also mentioned in Eliantes ebook on Casascius:
http://www.coinfirm.org/
To summarize all the drama:
There was two user here on bitcointalk that ended up having a 25 BTC Casascius coin with identical public addy (what a nightmare that must have been).
Mike Caldwell himself determined fairly easy which one that was fake, since this particular coin was never sold from Casascius, Mike gave it as a present to one of his close friends, and that friend had later sold it to the user on this forum.
I am not aware of any cases since this one.