Is performing a 51% attack / double spend on Bitcoin illegal? Suppose I had an ultra large ASIC from the future, and could perform a double spend on the core Bitcoin network. At this point in Bitcoin's development, it is no longer a game - there's real money involved.

Would performing a successful double spend be considered illegal (e.g. in the U.S / Europe)?
A double-spend would be blackletter law illegal, guys.  Fraud and theft include a wide variety of intentional deception wrongfully depriving someone of property - use of a government-minted currency is not, and never was, a required element.  Look at any larceny or fraud statute - it will refer to loss of or damage to person and property not "dollars."

Schwarz - when you double-spend, you are representing to the seller as an existing fact that you are passing a valid bitcoin, which is a misrepresentation.  Your representations as to fact are readily, and usually, implied from conduct in fraud and "larceny by trick." You don't have to literally state them in words.  this goes for more than bitcoin.  If you were considering stealing puppies to return them for a reward, it's a legal problem for the same reason.  But do I really have to tell you this?

Of course, these are just general legal notes, and not legal advice.  Before you consider undertaking any questionable business activity, consult an attorney.