How would you want to handle the problem of rounding?

Not that I think it will have a lot of financial significance, but it is possible to get into situations where multiple txouts could yield slightly different total value depending how they are spent.

There is also the problem of agreeing on time, and even the time between when a transaction is issued and when it is included in the block chain could make a transaction over-spend its (now shrunken) inputs, or create other sorts of "slightly off" errors.  It might make sense to define the demurrage time in terms of the block chain and not in terms of 'wall clock' time.

An alternative is to have the underlying implementation store non-shrinking coins as they are now, with no demurrage, and to change the user interface to map a nominal demur-coin to the underlying non-shrinking coin according to a formula.  Then the underlying implementation would be represented in terms of coins-at-a-particular-point-in-time which are converted to present nominal demur-coins by a pre-determined time-dependent ratio.  There is still the problem of 'slightly off' errors if someone is asking 1 DMC in payment and you make the 1 DMC payment but by the time they receive it they get only 0.99999999 DMC.  If this is a human recipient it makes no difference but an automated payment processor could reject it.