Think client-side certificate or OAuth token.

I have username/password/yubikey combination to log into LastPass. On my Android phone though, I only need my username/password to access LastPass.

The trust lies with LastPass being able to uniquely identify my device as mine, and not someone spoofing me.

Yubikey is soley used from PC where I can plug in the yubikey. I could also have trusted computers, but I prefer to keep trusted devices to a minimum.

Not yet, though they do have a version that works with Android over NFC.