The new ssl pinning proposals work very similarly to ssh. When the user visits an https site with pinning enabled, the browser will remember the server's public

The new ssl pinning proposals work very similarly to ssh. When the user visits an https site with pinning enabled, the browser will remember the server's public key similar to ssh's known_hosts. You're correct that this won't protect against a MITM during this first visit and will cause confusion if an attacker pins his fraudulent public key. However, I would argue that pinning provides a large net security gain in practice. The same model of vulnerable initial visit but secure subsequent visits is used in HSTS to prevent ssl stripping attacks.