I do not see what that "something" is, which makes it a ponzi in my eyes. (An open, honest ponzi, but a ponzi nevertheless)
> If a bank lend out x times the value of the bitcoin you deposited, they will not be able to pay them out as bitcoins.
But of course, if x < 1 (as is in fractional reserve banking), the bank can pay out bitcoins.
> If you dont have access to your bitcoins as bitcoins, but only as a bank statement about how much they owe you, you effectivly traded your bitcoins to an IOU.
Exactly. And if that IOU is from a credible enough institution, there is no reason[1] why I would differentiate taking a payment in that IOU or bitcoin. In which case that IOU is bitcoin-denominated currency and the amount of bitcoin-denominated currency in circulation is no more constant.
[1]Well, actually there may be at least two reasons to differentiate. First, the IOU may pay interest unlike bitcoin. Second, the practicalities in transacting bitcoins or IOU's may differ, which in all probability dictate that I prefer the IOU...)