I too have recently switched to the new metal AMEX Platinum Charge card. I thought I might have been the only one with these PIN/Sign "issues" but obviously not.
One of the first transactions I made on the new card was $900 at a smaller store in Cairns. No sign, no PIN. An hour or so later $25 in a bookshop and "Signature required". Most larger stores (Coles, Woolworths, Myer, DJ's) seem to be as usual (under $100 - no PIN, over $100 - PIN). There really doesn't seem to be much logic to it.
I also called AMEX but was told that the software in the machines were being updated for the new metal cards and it would take a while for it to go out across the network. In the interim expect to have to sign in some shops.
Tallfont