I do not understand this move. If blockchain size is an issue, I don't understand why the focus isn't pruning the blockchain so that only the hashed headers are stored for past transactions. This defeats the point of having 8 decimal places if you can only reasonably use 3+.
It's presumably only temporary measure, and only present in the reference client. Anything below four decimal places is completely worthless at this point anyway; do you really have a use for being able to send 0.01c USD?
Placing the constraint right on the edge of the current value seems like it will just cause a lot of problems as the value continues to fluctuate.