donate them to pirate's scam victims :') you will be forever known as a hero in bitcoinland!

That's a lot of butter.

I was considering that, now done. Part 2 would be to transfer the 519.704, now 519.70399999 because of the minimum trans fee to segregate the two amounts, to an offline wallet that I have secured against fire/flood/theft. Then if someone had a collision, they'll see "their" key emptied, and hopefully post in this forum that they suffered a theft (if they don't see this thread first), sign with MY key, and get their BTC back. Then I'll have to generate a new vanity address knowing 1tbzj is compromised.

Strangeness?

This doesn't actually make brain wallets safe.

Free $7,000?

Lucky you.

it's not a collision.

someone made a mistake (accidentally entered the address somewhere, at an exchange for example. It can happen easily: have 1btc in clipboard for some reason, try to copy own address and fail, paste into mtgox, forget to double-check). That's the only plausible explanation I can come up with.

My take on what is the "right" thing to do: Wait (maybe 3 months, it's up to you). If someone shows up and can prove he owns the address, give him the money. Expect 10% or so in "finder's reward". If noone shows up, you can consider the money yours... you just won the lottery. It's up to you what to do with it, but if you want to build reputation by not keeping it for yourself consider helping some bitcoin-related project or starting your own. Don't give it as pittance to scam victims, that'd be just stupid and probably throwing money down the same hole. Put up a bounty for something cool you'd like to see done or whatever and don't let anyone else (including me) tell you what to do

Does anyone know what our beloved law has to say about such a case (say someone accidentally mails you 4 ounces of gold, no sending address on the package). You're probably obligated to make reasonable effort to return it, I'm not sure. After that? I have no idea.

What do you mean?

1. You've posted on the forums. Wait for 3 months and see if anyone claims it.
2. If not, take the coins. Donate it if you're generous.
3. Feel good karma or feel profit

I am not a client developer but the simple answer is no.   Private keys are randomly generated and no client has a mechanism of knowing what other addresses have been created.  For example if you wanted to you could generate a private key by rolling a large number of dice and converting the results into a 256 bit number, then manually compute (using pen and a lot of paper) the public key and address.  It is simply impossible for a node to know what other random numbers in the world have been used.

Still I don't believe this is a collision, not without extensive evidence to the contrary. Assuming the private key is randomly generated, the odds of a collision are simple so incredibly low that they can be considered ~0% for all intents and purposes.