molecular:
can the Trezor ppl see our balances and tx's while the Trezor is logged into myTrezor.com?
yes, unfortunately. At least they're not using your xpub key to transfer addresses to watch, so they don't know your future keys (except the 5 or so per account they scan ahead).
That's my biggest criticism with myTrezor and it's why I switched to electrum as soon as the development code had trezor working. It's not perfect with electrum, either, but better (I don't know exactly what, but they do some stuff to increase privacy towards server operators).
I went to the hassle of running my own electrum server (public, of course, so I can hide in the masses when broadcasting transactions). Feel free to use it (electrum.0x0000.de), I'm not logging anything or looking at the traffic in any way.
How could they overlook such an obvious privacy violation when designing the wallet? It should be a local client that broadcasts tx's out to the network.