1 - I chose a hologram with the name Casascius on it, because that would be prohibitively expensive ($thousands) for someone else to duplicate.  And if they're going to go to the effort, they may as well make their own legitimate coins (clearly there's a demand) rather than perpetrating a fraud that would get quickly found out.

2 - Each coin has the first 8 characters of the bitcoin address on it, which is enough for Block Explorer to find it.  When I load bitcoins on coins, the transactions are fairly distinct - lots of outputs of 1 BTC to lots of addresses wrapped into a single transaction.  So if someone were to vanitygen one (the odds of it happening purely at random are 2.2 trillion to 1) you could find the right transaction.

3 - it becomes a piece of metal, not quite as desirable, but still cool nonetheless.

4 - I could probably put together QR codes, but I favor readable codes for several good reasons, the most significant being that most people don't know how to scan them into a program without retyping them anyway (typical joe will scan with his iphone and type into his computer what he sees on the screen, defeating the purpose).  And second, if the code gets torn or damaged, your eyeball will do a much better job of piecing it back together than a phone that will just say it can't read the code.

5 - Probably no smaller amounts.  Maybe bigger amounts (such as 10 or 50 BTC in larger, more ornate coins).  This is fairly time consuming for me as it is, and I fear that demand could outstrip my ability to supply them.  It's hard to want to delegate it, as I can make sure that all of the codes are legible and correct and match the address on the outside, but somebody else might not take that so seriously.

6 - Others have asked about durability.  I don't know yet - haven't had them that long.  The adhesive is quite strong and they will probably resist water just fine, but what would really ruin them is if they sat in your pocket and got jingled around with your keys.  I would treat them as delicate until proven otherwise.  I am experimenting with spraying on a layer of polyacrylate which adds significant protection to the hologram, particularly at the edges, but it will take me several days to really know for sure how that will end up.  Seriously though, I hope somebody copies my idea and does a better job and makes a wash-proof key-proof bullet-proof coin for less